Ship form-associated custom elements

Intent to ship: https://groups.google.com/a/chromium.org/d/msg/blink-dev/0w_Z-9_kSNs/QV6ChIapAwAJ

Bug: 905922
Change-Id: Ic5bec9cab0f56e8a95684896536eabb0205d5c16
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636990Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665053}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -478,6 +478,7 @@
     {
       // http://crbug.com/905922
       name: "ElementInternals",
+      status: "stable",
       implied_by: ["FormAssociatedCustomElements"],
     },
     {
@@ -628,7 +629,7 @@
     },
     {
       name: "FormAssociatedCustomElements",
-      status: "experimental",
+      status: "stable",
     },
     // This is to communicate features::FormControlsRefresh from ui (and can be
     // removed when the feature launches).

third_party/blink/web_tests/virtual/stable/webexposed/element-instance-property-listing-expected.txt

@@ -30,6 +30,7 @@ namespace http://www.w3.org/1999/xhtml
     property append
     property appendChild
     property assignedSlot
+    property attachInternals
     property attachShadow
     property attributeStyleMap
     property attributes

third_party/blink/web_tests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -1692,6 +1692,18 @@ interface Element : Node
     setter scrollLeft
     setter scrollTop
     setter slot
+interface ElementInternals
+    attribute @@toStringTag
+    getter form
+    getter labels
+    getter validationMessage
+    getter validity
+    getter willValidate
+    method checkValidity
+    method constructor
+    method reportValidity
+    method setFormValue
+    method setValidity
 interface EnterPictureInPictureEvent : Event
     attribute @@toStringTag
     getter pictureInPictureWindow
@@ -2266,6 +2278,7 @@ interface HTMLElement : Element
     getter tabIndex
     getter title
     getter translate
+    method attachInternals
     method blur
     method click
     method constructor