Implement autoplay gesture override experiment.

Source/core/html/AutoplayExperimentHelper.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef AutoplayExperimentHelper_h
+#define AutoplayExperimentHelper_h
+
+#include "core/html/AutoplayExperimentConfig.h"
+#include "core/page/Page.h"
+#include "platform/Timer.h"
+#include "platform/geometry/IntRect.h"
+
+namespace blink {
+class Document;
+class HTMLMediaElement;
+class EventListener;
+
+// 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.
+    AutoplayPaused = 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 detected, but the gesture
+    // requirement was overridden by experiment".  They do not include cases
+    // where no user gesture is required (!m_userGestureRequiredForPlay).
+
+    // User gesture requirement was bypassed, and playback started, during
+    // initial load via the autoplay flag.  If playback was deferred due to
+    // visibility requirements, then this does not apply.
+    GesturelessPlaybackStartedByAutoplayFlagImmediately = 6,
+
+    // User gesture requirement was bypassed, and playback started, when
+    // the play() method was called.  If playback is deferred due to
+    // visibility requirements, then this does not apply.
+    GesturelessPlaybackStartedByPlayMethodImmediately = 7,
+
+    // User gesture requirement was bypassed, and playback started, after
+    // an element with the autoplay flag moved into the viewport.  Playback
+    // was deferred earlier due to visibility requirements.
+    GesturelessPlaybackStartedByAutoplayFlagAfterScroll = 8,
+
+    // User gesture requirement was bypassed, and playback started, after
+    // an element on which the play() method was called was moved into the
+    // viewport.  Playback had been deferred due to visibility requirements.
+    GesturelessPlaybackStartedByPlayMethodAfterScroll = 9,
+
+    // play() failed to play due to gesture requirement.
+    PlayMethodFailed = 10,
+
+    // Some play, whether user initiated or not, started.
+    AnyPlaybackStarted = 11,
+    // Some play, whether user initiated or not, paused.
+    AnyPlaybackPaused = 12,
+    // Some playback, whether user initiated or not, bailed out early.
+    AnyPlaybackBailout = 13,
+
+    // This enum value must be last.
+    NumberOfAutoplayMetrics,
+};
+
+class AutoplayExperimentHelper {
+public:
+    AutoplayExperimentHelper(HTMLMediaElement&);
+    ~AutoplayExperimentHelper();
+
+    void becameReadyToPlay();
+    void playMethodCalled();
+    void pauseMethodCalled();
+    void mutedChanged();
+    void positionChanged();
+
+private:
+    // 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 isEligible() const;
+
+    // Set the muted flag on the media if we're in an experiment mode that
+    // requires it, else do nothing.
+    void muteIfNeeded();
+
+    // Maybe override the requirement for a user gesture, and start playing
+    // autoplay media.  Returns true if only if it starts playback.
+    bool maybeStartPlaying();
+
+    // 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 prepareToPlay(AutoplayMetrics);
+
+    // Return our media element's document.
+    Document& document() const;
+
+    inline bool enabled(AutoplayExperimentConfig::Mode mode) const
+    {
+        return ((int)m_mode) & ((int)mode);
+    }
+
+private:
+    HTMLMediaElement& m_element;
+
+    AutoplayExperimentConfig::Mode m_mode;
+
+    // Autoplay experiment state.
+    // True if we've received a play() without a pause().
+    bool m_playPending : 1;
+
+    friend class Internals;
+};
+
+} // namespace blink
+
+#endif // AutoplayExperimentHelper_h