Autoplay: use an autoplay policy setting in Blink.

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: "mediaPlaybackRequiresUserGesture",
-      initial: false,
-    },
-    {
       name: "mediaPlaybackGestureWhitelistScope",
       type: "String",
     },
 
-    // This flags overrides mediaPlaybackRequiresUserGesture
-    {
-      name: "crossOriginMediaPlaybackRequiresUserGesture",
-      initial: false,
-    },
-
     {
       name: "presentationRequiresUserGesture",
       initial: true,
     },
 
     {
       name: "embeddedMediaExperienceEnabled",
       initial: false,
     },
 
@@ skipping 677 matching lines @@
       initial: true,
       invalidate: "MediaControls",
     },
 
     // Whether we should not update selection attributes when mutating selection range.
     // TODO(changwan): remove this flag when we no longer support Android M.
     {
       name: "doNotUpdateSelectionOnMutatingSelectionRange",
       initial: false,
     },
+
+    // Defines the autoplay policy to use.
+    {
+      name: "autoplayPolicy",
+      type: "AutoplayPolicy::Type",
+      initial: "AutoplayPolicy::Type::kNoUserGestureRequired",
+    },
   ],
 }