Implement autoplay gesture override experiment.

Source/core/html/HTMLMediaElement.h

Index: Source/core/html/HTMLMediaElement.h
diff --git a/Source/core/html/HTMLMediaElement.h b/Source/core/html/HTMLMediaElement.h
index 2f1bea3fc8f96c6333f9c3c7361d47bcf86982df..9e98603a4c3a36ae444441fada49feb63cb78b03 100644
--- a/Source/core/html/HTMLMediaElement.h
+++ b/Source/core/html/HTMLMediaElement.h
@@ -29,6 +29,7 @@
 #include "core/CoreExport.h"
 #include "core/dom/ActiveDOMObject.h"
 #include "core/events/GenericEventQueue.h"
+#include "core/html/AutoplayExperimentHelper.h"
 #include "core/html/HTMLElement.h"
 #include "core/html/track/TextTrack.h"
 #include "platform/Supplementable.h"
@@ -390,7 +391,13 @@ private:
     // This does not check user gesture restrictions.
     void playInternal();
 
-    void gesturelessInitialPlayHalted();
+    // If we are about to enter a stopped state, call this to record

[philipj_slow, 2015/08/13 10:15:40]

s/stop/pause/, media elements don't really have a concept of being stopped, and
it's easy to confuse this with HTMLMediaElement::stop(), which is something else
entirely, for ActiveDOMObject.

[liberato (no reviews please), 2015/09/01 06:54:20]

updated, and the metric names.  was trying not to modify the old enum names, but
what the heck.

+    // autoplay metrics.  If we were already in a stopped state, then
+    // this does nothing.
+    void recordMetricsIfStopping();
+    // Could stopping at this point be considered a bailout of playback?
+    // (as in, "The user really didn't want to play this").
+    bool isBailout() const;
     void autoplayMediaEncountered();
     void allowVideoRendering();
 
@@ -426,6 +433,8 @@ private:
     bool isBlockedOnMediaController() const;
     bool isAutoplaying() const { return m_autoplaying; }
 
+    void recordAutoplayMetric(AutoplayMetrics);
+
     WebMediaPlayer::CORSMode corsMode() const;
 
     // Returns the "direction of playback" value as specified in the HTML5 spec.
@@ -445,6 +454,12 @@ private:
     // transition to HAVE_METADATA.
     void selectInitialTracksIfNecessary();
 
+    // Return true if and only if we require a user gesture before letting
+    // the media play.
+    bool isUserGestureRequiredForPlay() const;
+    void removeUserGestureRequirement();
+    void setInitialPlayWithoutUserGestures(bool);
+
     void audioTracksTimerFired(Timer<HTMLMediaElement>*);
 
     Timer<HTMLMediaElement> m_loadTimer;
@@ -617,6 +632,9 @@ private:
 
     friend class Internals;
     friend class TrackDisplayUpdateScope;
+    friend class AutoplayExperimentHelper;
+
+    AutoplayExperimentHelper m_autoplayHelper;
 
     static URLRegistry* s_mediaStreamRegistry;
 };