Media Controls: make the MediaControlsOverlayPlayButton a runtime enabled flag.

third_party/WebKit/Source/core/frame/Settings.json5

 {
   // Defines properties which are available on the Settings object.
   //
   // Please think carefully before adding a new Setting.  Some questions to
   // consider are:
   // - Should this be a RuntimeEnabledFeature instead? Settings are for things
   //   which we support either values of at runtime.  Features are set at renderer
   //   process startup and are never changed. Features also tend to be set to a
   //   value based on the platform or the stability of the code in question, where
   //   as settings both codepaths need to be stable.
@@ skipping 198 matching lines @@
     {
       name: "hyperlinkAuditingEnabled",
       initial: false,
     },
     {
       name: "allowRunningOfInsecureContent",
       initial: true,
     },
 
     {
-      name: "mediaControlsOverlayPlayButtonEnabled",
-      initial: false,
-    },
-    {
       name: "mediaPlaybackRequiresUserGesture",
       initial: false,
     },
     {
       name: "mediaPlaybackGestureWhitelistScope",
       type: "String",
     },
 
     // This flags overrides mediaPlaybackRequiresUserGesture
     {
@@ skipping 690 matching lines @@
     },
 
     // Whether Blink should show media controls when `controls` attribute is used.
     {
       name: "mediaControlsEnabled",
       initial: true,
       invalidate: "MediaControls",
     },
   ],
 }