Enable user gesture requirement for autoplay experiment.

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

Index: third_party/WebKit/Source/core/html/AutoplayExperimentHelper.h
diff --git a/third_party/WebKit/Source/core/html/AutoplayExperimentHelper.h b/third_party/WebKit/Source/core/html/AutoplayExperimentHelper.h
index 8db55f9e4f450e0a44ace99e665b489d81435cc2..ee05829aad9eea5b05fb7287af9e02a729fbd854 100644
--- a/third_party/WebKit/Source/core/html/AutoplayExperimentHelper.h
+++ b/third_party/WebKit/Source/core/html/AutoplayExperimentHelper.h
@@ -151,6 +151,10 @@ public:
     void playbackStopped();
     void initialPlayWithUserGesture();
 
+    // Returns true if and only if any experiment is enabled (i.e., |m_mode|
+    // is not ExperimentOff).
+    bool isExperimentEnabled();
+
     // Clean up.  For Oilpan, this means "early in HTMLMediaElement's dispose".
     // For non-Oilpan, just delete the object.
     void dispose();