Add UseCounter for dynamic delegation of autoplay

We want to put the feature of "dynamic delegation of autoplay" in a
origin trial, and we need to add a UMA metrics to track this feature's
usage. So we add a new feature kAutoplayDynamicDelegation.

Bug: 985914
Change-Id: Icabf1fd996967e36deb6f3c00bee26c4473b7ddf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1717241
Commit-Queue: Lan Wei <lanwei@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681113}

third_party/blink/public/mojom/web_feature/web_feature.mojom

@@ -2358,6 +2358,7 @@ enum WebFeature {
   kDownloadInAdFrame = 2974,
   kDownloadInSandbox = 2975,
   kDownloadWithoutUserGesture = 2976,
+  kAutoplayDynamicDelegation = 2977,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

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

@@ -2277,6 +2277,8 @@ void WebLocalFrameImpl::DispatchMessageEventWithOriginCheck(
        msg_event->transferUserActivation()) ||
       msg_event->allowAutoplay()) {
     GetFrame()->TransferUserActivationFrom(source_frame);
+    if (msg_event->allowAutoplay())
+      UseCounter::Count(GetDocument(), WebFeature::kAutoplayDynamicDelegation);
   }
 
   GetFrame()->DomWindow()->DispatchMessageEventWithOriginCheck(

tools/metrics/histograms/enums.xml

@@ -24141,6 +24141,7 @@ Called by update_net_error_codes.py.-->
   <int value="2974" label="DownloadInAdFrame"/>
   <int value="2975" label="DownloadInSandbox"/>
   <int value="2976" label="DownloadWithoutUserGesture"/>
+  <int value="2977" label="AutoplayDynamicDelegation"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">