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 2bb0dd6888960b964924babfdecd28d794934ca2..edfc9d8701301a22bccbbc9785a1b4f866c9318c 100644
--- a/Source/core/html/HTMLMediaElement.h
+++ b/Source/core/html/HTMLMediaElement.h
@@ -287,6 +287,35 @@ protected:
     void setControllerInternal(MediaController*);
 
 private:
+    // These values are used for a histogram. Do not reorder.
+    enum AutoplayMetrics {
+        // Media element with autoplay seen.
+        AutoplayMediaFound = 0,
+        // Autoplay enabled and user stopped media play at any point.
+        AutoplayStopped = 1,
+        // Autoplay enabled but user bailed out on media play early.
+        AutoplayBailout = 2,
+        // Autoplay disabled but user manually started media.
+        AutoplayManualStart = 3,
+        // Autoplay was (re)enabled through a user-gesture triggered load()
+        AutoplayEnabledThroughLoad = 4,
+        // Autoplay disabled by sandbox flags.
+        AutoplayDisabledBySandbox = 5,
+        // AutoplayStopped for a media play that was started by the experiment.
+        AutoplayExperimentStopped = 6,
+        // AutoplayBailout for media a play that was started by the experiment.
+        AutoplayExperimentBailout = 7,
+        // Autoplay started by experiment when media scrolled into view.  We don't
+        // record whether it was a javascript or attribute autoplay request.
+        AutoplayExperimentStartedByScroll = 8,
+        // Autoplay started by experiment during initial load.
+        AutoplayExperimentStartedByLoad = 9,
+        // Autoplay started by experiment in play() call.
+        AutoplayExperimentStartedByPlay = 10,
+        // This enum value must be last.
+        NumberOfAutoplayMetrics,
+    };
+
     void resetMediaPlayerAndMediaSource();
 
     bool alwaysCreateUserAgentShadowRoot() const final { return true; }
@@ -427,6 +456,47 @@ private:
     bool isBlockedOnMediaController() const;
     bool isAutoplaying() const { return m_autoplaying; }
 
+    void recordAutoplayMetric(AutoplayMetrics);
+
+    // vvvv Helpers for clank autoplay investigation vvvv
+
+    // Install an event listener to check for changes in visibility.  If a
+    // listener is already installed, then this does nothing.
+    void autoplayExperimentInstallEventListenerIfNeeded();
+
+    // Remove any event listener.  It's okay to call this if one isn't
+    // installed already.
+    void autoplayExperimentClearEventListenerIfNeeded();
+
+    // Return true if any only if this player meets (most) of the eligibility
+    // requirements for the experiment to override the need for a user
+    // gesture.  This includes everything except the visibility test.
+    bool autoplayExperimentIsEligible() const;
+
+    // Return true if and only if the player is visible.
+    bool autoplayExperimentIsVisible();
+
+    // Set the mute flag on the media if we're in an experiment mode that
+    // requires it, else do nothing.
+    void autoplayExperimentMuteIfNeeded();
+
+    // Maybe override the requirement for a user gesture, and start playing
+    // autoplay media.  Returns true if only if it starts playback.
+    bool autoplayExperimentMaybeStartPlaying();
+
+    // Configure internal state to record that the autoplay experiment is
+    // going to start playback.  This doesn't actually start playback, since
+    // there are several different cases.
+    void autoplayExperimentPrepareToPlay(AutoplayMetrics);
+
+    // Begin (or start over) a periodic check for visibility.  We will poll
+    // during this check to see if the video is in view.
+    void beginPeriodicVisibilityCheck();
+
+    // Process a timer for checking visibility.
+    void visibilityTimerFired(Timer<HTMLMediaElement>*);
+    // ^^^^ Helpers for clank autoplay investigation ^^^^
+
     WebMediaPlayer::CORSMode corsMode() const;
 
     // Returns the "direction of playback" value as specified in the HTML5 spec.
@@ -550,6 +620,29 @@ private:
     bool m_initialPlayWithoutUserGestures : 1;
     bool m_autoplayMediaCounted : 1;
 
+    // Autoplay experiment state.
+    // True if we've received a play() without a pause().
+    bool m_autoplayExperimentPlayPending : 1;
+
+    // Autoplay experiment state.
+    // True if and only if we initiated playback because of the autoplay
+    // experiment.  Once set, this is never unset.
+    bool m_autoplayExperimentStartedByExperiment : 1;
+
+    // Autoplay experiment state.
+    // Touch listener for the autoplay experiment.
+    class AutoplayExperimentTouchListener;
+    friend class AutoplayExperimentTouchListener;
+    RefPtrWillBeMember<EventListener> m_autoplayExperimentTouchListener;
+
+    enum AutoplayExperimentMode {
+        ExperimentOff,
+        ExperimentAlways,
+        ExperimentIfMuted,
+        ExperimentPlayMuted
+    };
+    AutoplayExperimentMode m_autoplayExperimentMode;
+
     RefPtrWillBeMember<AudioTrackList> m_audioTracks;
     RefPtrWillBeMember<VideoTrackList> m_videoTracks;
     RefPtrWillBeMember<TextTrackList> m_textTracks;
@@ -613,6 +706,11 @@ private:
     AudioSourceProviderImpl m_audioSourceProvider;
 #endif
 
+    Timer<HTMLMediaElement> m_autoplayVisibilityTimer;
+    double m_autoplayLastScrollX;
+    double m_autoplayLastScrollY;
+    double m_autoplayVisibilityTimerSpan;
+
     friend class MediaController;
     PersistentWillBeMember<MediaController> m_mediaController;