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 ea7a3ed99e5c0cd3e0041999a27df6aaaec74161..0168c812720efcab9b65b00c3cb020b296c1fa62 100644
--- a/third_party/WebKit/Source/core/html/AutoplayExperimentHelper.h
+++ b/third_party/WebKit/Source/core/html/AutoplayExperimentHelper.h
@@ -154,6 +154,10 @@ 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();