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..8d4404087665221149e6239e48099ae86242dcc5 100644
--- a/Source/core/html/HTMLMediaElement.h
+++ b/Source/core/html/HTMLMediaElement.h
@@ -286,6 +286,46 @@ 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,
+
+        // These metrics indicate "no gesture but gesture requirement was
+        // overridden by experiment".  They do not include cases where no
+        // user gesture is required (!m_userGestureRequiredForPlay).
+        // Gestureless playback when media scrolled into view.  We don't
+        // record whether it was a javascript or attribute autoplay request.
+        GesturelessPlaybackStartedByScroll = 6,
+        // Autoplay started by experiment override during initial load.
+        GesturelessPlaybackStartedByLoad = 7,
+        // Autoplay started by experiment override in play() call.
+        GesturelessPlaybackStartedByPlayMethod = 8,
+
+        // play() failed to play due to gesture requirement.
+        PlayMethodFailed = 9,
+
+        // Some play, whether user initiated or not, started.
+        AnyPlaybackStarted = 10,
+        // Some play, whether user initiated or not, stopped.
+        AnyPlaybackStopped = 11,
+        // Some playback, whether user initiated or not, bailed out early.
+        AnyPlaybackBailout = 12,
+
+        // This enum value must be last.
+        NumberOfAutoplayMetrics,
+    };
+
     void resetMediaPlayerAndMediaSource();
 
     bool alwaysCreateUserAgentShadowRoot() const final { return true; }
@@ -390,7 +430,13 @@ 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
+    // 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 +472,47 @@ 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 autoplayExperimentIsInViewportIfNeeded();
+
+    // 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 the viewport.
+    void notifyScrolled();
+
+    // Process a timer for checking visibility.
+    void viewportTimerFired(Timer<HTMLMediaElement>*);
+    // ^^^^ Helpers for clank autoplay investigation ^^^^
+
     WebMediaPlayer::CORSMode corsMode() const;
 
     // Returns the "direction of playback" value as specified in the HTML5 spec.
@@ -554,6 +641,31 @@ private:
     PersistentWillBeMember<TextTrackList> m_textTracks;
     PersistentHeapVectorWillBeHeapVector<Member<TextTrack>> m_textTracksWhenResourceSelectionBegan;
 
+    // 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        = 0,
+        ExperimentEnabled    = 1 << 0,
+        ExperimentIfViewport = 1 << 1,
+        ExperimentIfMuted    = 1 << 2,
+        ExperimentIfMobile   = 1 << 3,
+        ExperimentPlayMuted  = 1 << 4
+    };
+    int m_autoplayExperimentMode; // Bitwise-or of AutoplayExperimentMode
+
     OwnPtrWillBeMember<CueTimeline> m_cueTimeline;
 
 #if ENABLE(WEB_AUDIO)
@@ -612,6 +724,11 @@ private:
     AudioSourceProviderImpl m_audioSourceProvider;
 #endif
 
+    Timer<HTMLMediaElement> m_autoplayViewportTimer;
+    double m_autoplayLastScrollX;
+    double m_autoplayLastScrollY;
+    double m_autoplayViewportTimerSpan;
+
     friend class MediaController;
     PersistentWillBeMember<MediaController> m_mediaController;