When autoplaying muted video, record when unmute happen and the result.

third_party/WebKit/Source/core/html/HTMLMediaElement.h

Index: third_party/WebKit/Source/core/html/HTMLMediaElement.h
diff --git a/third_party/WebKit/Source/core/html/HTMLMediaElement.h b/third_party/WebKit/Source/core/html/HTMLMediaElement.h
index 282b0fae379e98ae19c87e05ddba06d8669e3091..9863e39251d31afb640160653852f921101638df 100644
--- a/third_party/WebKit/Source/core/html/HTMLMediaElement.h
+++ b/third_party/WebKit/Source/core/html/HTMLMediaElement.h
@@ -298,6 +298,13 @@ protected:
     void recordAutoplayMetric(AutoplayMetrics);
 
 private:
+    // These values are used for histograms. Do not reorder.
+    enum AutoplayUnmuteActionStatus {
+        AutoplayUnmuteActionFailure = 0,
+        AutoplayUnmuteActionSuccess = 1,
+        AutoplayUnmuteActionMax
+    };
+
     void resetMediaPlayerAndMediaSource();
 
     bool alwaysCreateUserAgentShadowRoot() const final { return true; }
@@ -493,6 +500,7 @@ private:
     EnumerationHistogram& showControlsHistogram() const;
 
     void recordAutoplaySourceMetric(int source);
+    void recordAutoplayUnmuteStatus(AutoplayUnmuteActionStatus);
 
     void onVisibilityChangedForAutoplay(bool isVisible);