Standardize the condition to open popup with window.open

Fixes whatwg#5872

Define an "is popup" member in top-level browsing context, and set it to true
only when the browsing context is created by calling `window.open` with `width`
features provided.

Also add non-normative notes/example for using popup UI depending on "is popup"
member.

source

@@ -77889,6 +77889,23 @@ popup4.close();</code></pre></div>
   <p>A <span>top-level browsing context</span> has an associated <dfn data-x="tlbc
   group">group</dfn> (null or a <span>browsing context group</span>). It is initially null.</p>
 
+  <p>A <span>top-level browsing context</span> has an <dfn>is popup</dfn>
+  boolean. It is initially false.</p>
+
+  <p class="note">User agent <!--non-normative-->can use <span>is popup</span>
+  value to decide what kind of web browser interface to provide for the
+  <span>browsing context</span>.</p>
+
+  <p class="example">For example, user agent <!--non-normative-->can provide
+  minimal UI if <span>is popup</span> is true. In the minimal UI, it's
+  <!--non-normative-->recommended not to hide the browser interface element
+  that displays the <span>URL</span> or the domain of the
+  <span>active document</span>.</p>
+
+  <p class="note">User agent <!--non-normative-->can use <span>is popup</span>
+  value to decide whether to perform the <!--non-normative-->optional steps in
+  <span>set up browsing context features</span> steps or not.</p>
+
   <p>It is possible to create new browsing contexts that are related to a <span>top-level browsing
   context</span> while their <span data-x="bc-container">container</span> is null. Such browsing
   contexts are called <dfn data-x="auxiliary browsing context" data-lt="auxiliary browsing context"
@@ -79239,9 +79256,51 @@ dictionary <dfn>WindowPostMessageOptions</dfn> : <span>PostMessageOptions</span>
 
    <li><p>If <var>target browsing context</var> is null, then return null.</p></li>
 
-   <li><p>If <var>new</var> is true, then <span>set up browsing context features</span> for
-   <var>target browsing context</var> given <var>tokenizedFeatures</var>. <ref
-   spec="CSSOMVIEW"></p></li>
+   <li>
+    <p>If <var>new</var> is true, then:</p>
+    <ol>
+     <li>
+      <p>If <var>tokenizedFeatures</var>["<code data-x="">width</code>"]
+      <span data-x="map exists">exists</span>, then:</p>
+      <ol>
+       <li><p>Let <var>isPopupRequested</var> be true.</li>
+      </ol>
+     </li>
+     <li>
+      <p>Otherwise:</p>
+      <ol>
+       <li><p>Let <var>isPopupRequested</var> be false.</li>
+      </ol>
+     </li>
+     <li>
+      <p>If the user agent has been configured to override
+      <var>isPopupRequested</var>:</p>
+      <ol>
+       <li>
+        <p>Let <var>isPopup</var> be user-configured value, possibly depending
+        on <var>isPopupRequested</var>.</p>
+       </li>
+      </ol>
+     </li>
+     <li>
+      <p>Otherwise:</p>
+      <ol>
+       <li>
+        <p>Let <var>isPopup</var> be <var>isPopupRequested</var>.</p>
+       </li>
+      </ol>
+     </li>
+     <li>
+      <p>Set the <var>target browsing context</var>'s <a href="#is-popup">is
+      popup</a> to <var>isPopup</var>.</p>
+     </li>
+     <li>
+      <p><span>Set up browsing context features</span> for
+      <var>target browsing context</var> given
+      <var>tokenizedFeatures</var>. <ref spec="CSSOMVIEW"></p>
+     </li>
+    </ol>
+   </li>
 
    <li><p>Let <var>urlRecord</var> be the <span>URL</span> "<code>about:blank</code>".</p></li>