AMP: add AMP compat file and use in Jetpack features.

Summary:
After adding the AMP compat file that ships with Jetpack (this was done in D22313, D26814, D26819, D26921, and D26928), let's start using this class.

This replaces D22154 and D23501, and allows us to start syncing any future changes that would rely on the Jetpack_AMP_Support class.

Related Jetpack PRs:
- #10945
- #11195, which reverted some of the changes in the PR above.
- #12054
- #12053
- #12026

Discussion:
- https://[private link]

We'll need to test for issues like the ones that popped up on Jetpack at the time:
#11169

We will also need to account for the fact that WordPress.com does not run the latest version of the AMP plugin. It runs an old version (0.6.2 vs. the current version on W.org 1.0.2), with its own version of some of the compatibility fixes that come with the class we've added (see [[ https://[private link].php | here ]]) and needs to be updated as per the discussions here:
- https://[private link] (D14521)
- https://[private link]

Test Plan:
* Create a post with a gallery
* Add a Facebook sharing button to that post.
* Try Liking that post.
* Comment on one of the images in the gallery.
* Load the post in an AMP view, and repeat the steps below.

In all cases:
- You should not see any js errors in the browser console.
- You should not get any PHP notices in your debug log.

Reviewers: zinigor

Tags: #touches_jetpack_files

Differential Revision: D26821-code

This commit syncs r190790-wpcom.

modules/carousel/jetpack-carousel.php

@@ -153,7 +153,10 @@ function display_bail_message( $output = '' ) {
 	}
 
 	function check_if_shortcode_processed_and_enqueue_assets( $output ) {
-		if ( Jetpack_AMP_Support::is_amp_request() ) {
+		if (
+			class_exists( 'Jetpack_AMP_Support' )
+			&& Jetpack_AMP_Support::is_amp_request()
+		) {
 			return $output;
 		}
 
@@ -208,7 +211,10 @@ function check_if_shortcode_processed_and_enqueue_assets( $output ) {
 	 * @return string $content Post content.
 	 */
 	function check_content_for_blocks( $content ) {
-		if ( Jetpack_AMP_Support::is_amp_request() ) {
+		if (
+			class_exists( 'Jetpack_AMP_Support' )
+			&& Jetpack_AMP_Support::is_amp_request()
+		) {
 			return $content;
 		}
 
@@ -354,7 +360,10 @@ function enqueue_assets() {
 	}
 
 	function set_in_gallery( $output ) {
-		if ( Jetpack_AMP_Support::is_amp_request() ) {
+		if (
+			class_exists( 'Jetpack_AMP_Support' )
+			&& Jetpack_AMP_Support::is_amp_request()
+		) {
 			return $output;
 		}
 		$this->in_gallery = true;
@@ -372,7 +381,10 @@ function set_in_gallery( $output ) {
 	 * @return string Modified HTML content of the post
 	 */
 	function add_data_img_tags_and_enqueue_assets( $content ) {
-		if ( Jetpack_AMP_Support::is_amp_request() ) {
+		if (
+			class_exists( 'Jetpack_AMP_Support' )
+			&& Jetpack_AMP_Support::is_amp_request()
+		) {
 			return $content;
 		}
 
@@ -426,7 +438,10 @@ function add_data_img_tags_and_enqueue_assets( $content ) {
 	}
 
 	function add_data_to_images( $attr, $attachment = null ) {
-		if ( Jetpack_AMP_Support::is_amp_request() ) {
+		if (
+			class_exists( 'Jetpack_AMP_Support' )
+			&& Jetpack_AMP_Support::is_amp_request()
+		) {
 			return $attr;
 		}
 
@@ -498,7 +513,10 @@ function add_data_to_images( $attr, $attachment = null ) {
 
 	function add_data_to_container( $html ) {
 		global $post;
-		if ( Jetpack_AMP_Support::is_amp_request() ) {
+		if (
+			class_exists( 'Jetpack_AMP_Support' )
+			&& Jetpack_AMP_Support::is_amp_request()
+		) {
 			return $html;
 		}
 

modules/related-posts/jetpack-related-posts.php

@@ -1604,12 +1604,15 @@ protected function _log_click( $post_id, $to_post_id, $link_position ) {
 	 */
 	protected function _enabled_for_request() {
 		$enabled = is_single()
-			&&
-				! is_admin()
-			&&
-				( !$this->_allow_feature_toggle() || $this->get_option( 'enabled' ) )
-			&&
-				! Jetpack_AMP_Support::is_amp_request();
+			&& ! is_admin()
+			&& ( ! $this->_allow_feature_toggle() || $this->get_option( 'enabled' ) );
+
+		if (
+			class_exists( 'Jetpack_AMP_Support' )
+			&& Jetpack_AMP_Support::is_amp_request()
+		) {
+			$enabled = false;
+		}
 
 		/**
 		 * Filter the Enabled value to allow related posts to be shown on pages as well.

modules/sharedaddy/sharing-service.php

@@ -575,7 +575,10 @@ function sharing_maybe_enqueue_scripts() {
 }
 
 function sharing_add_footer() {
-	if ( Jetpack_AMP_Support::is_amp_request() ) {
+	if (
+		class_exists( 'Jetpack_AMP_Support' )
+		&& Jetpack_AMP_Support::is_amp_request()
+	) {
 		return;
 	}