Add experimental support for enterKeyHint.

enterKeyHint allows customization of the virtual keyboard enter
key. Enabled behind a flag since it isn't approved to ship yet.

Spec:
whatwg/html#3538

BUG=634330

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I32b510d8157e0c676300a38bb4c7fa2bbced0b3d

html/dom/elements-misc.js

@@ -53,6 +53,7 @@ var miscElements = {
 
   // Global attributes should exist even on unknown elements
   undefinedelement: {
+    enterKeyHint: {type: "enum", keywords: ["enter", "done", "go", "next", "previous", "search", "send"]},
     inputMode: {type: "enum", keywords: ["none", "text", "tel", "url", "email", "numeric", "decimal", "search"]},
   },
 };

html/semantics/forms/attributes-common-to-form-controls/contains.json

@@ -23,6 +23,10 @@
         "id": "autofocusing-a-form-control-the-autofocus-attribute",
         "original_id": "autofocusing-a-form-control:-the-autofocus-attribute"
     },
+    {
+        "id": "input-modalities-the-enterkeyhint-attribute",
+        "original_id": "input-modalities:-the-enterkeyhint-attribute"
+    },
     {
         "id": "input-modalities-the-inputmode-attribute",
         "original_id": "input-modalities:-the-inputmode-attribute"
@@ -31,4 +35,4 @@
         "id": "autofilling-form-controls-the-autocomplete-attribute",
         "original_id": "autofilling-form-controls:-the-autocomplete-attribute"
     }
-]
+]