Use ExperimentalAutoplayDynamicDelegation flag

We are adding a tentative API to experiment with dynamic delegation of
"autoplay" capability, and it will work with run-time flag
--enable-blink-features=ExperimentalAutoplayDynamicDelegation.

We have a test page with this API, which will auto-play the video after
click with the flag.
http://mustaqahmed.github.io/web/autoplay-dynamic-delegation-tester.html

Bug: 985914
Change-Id: I7756032bd8e18f55bc77fdb1929238ee724d8958
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1779012Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Lan Wei <lanwei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692296}

content/browser/frame_host/render_frame_proxy_host.cc

@@ -460,12 +460,18 @@ void RenderFrameProxyHost::OnRouteMessageEvent(
 
       // Transfer user activation state in the frame tree in the browser and
       // the non-source and non-target renderer processes when
-      // |transfer_user_activation| is true.
+      // |transfer_user_activation| is true. We are making an expriment with
+      // dynamic delegation of "autoplay" capability using this post message
+      // approach to transfer user activation.
       // TODO(lanwei): we should transfer user activation state only when
       // |source_rfh| and |target_rfh| are in the same frame tree.
-      if (base::FeatureList::IsEnabled(
+      bool should_transfer_user_activation =
+          base::FeatureList::IsEnabled(
               features::kUserActivationPostMessageTransfer) &&
-          message.transfer_user_activation &&
+          message.transfer_user_activation;
+      should_transfer_user_activation =
+          should_transfer_user_activation || message.allow_autoplay;
+      if (should_transfer_user_activation &&
           source_rfh->frame_tree_node()->HasTransientUserActivation()) {
         target_rfh->frame_tree_node()->TransferUserActivationFrom(source_rfh);
       }

third_party/blink/renderer/core/frame/dom_window.cc

@@ -493,11 +493,17 @@ void DOMWindow::DoPostMessage(scoped_refptr<SerializedScriptValue> message,
       user_activation, options->transferUserActivation(), allow_autoplay);
 
   // Transfer user activation state in the source's renderer when
-  // |transferUserActivation| is true.
+  // |transferUserActivation| is true. We are making an expriment with
+  // dynamic delegation of "autoplay" capability using this post message
+  // approach to transfer user activation.
   // TODO(lanwei): we should execute the below code after the post task fires
   // (for both local and remote posting messages).
-  if (RuntimeEnabledFeatures::UserActivationPostMessageTransferEnabled() &&
-      options->transferUserActivation() &&
+  bool should_transfer_user_activation =
+      RuntimeEnabledFeatures::UserActivationPostMessageTransferEnabled() &&
+      options->transferUserActivation();
+  should_transfer_user_activation =
+      should_transfer_user_activation || allow_autoplay;
+  if (should_transfer_user_activation &&
       LocalFrame::HasTransientUserActivation(source_frame)) {
     GetFrame()->TransferUserActivationFrom(source_frame);