Implement autoplay gesture override experiment.

Source/core/html/AutoplayExperimentConfig.h

Index: Source/core/html/AutoplayExperimentConfig.h
diff --git a/Source/core/html/AutoplayExperimentConfig.h b/Source/core/html/AutoplayExperimentConfig.h
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index 0000000000000000000000000000000000000000..e1dd6daa4ca8ae2d216aa7a2eb938f05be704f31
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+++ b/Source/core/html/AutoplayExperimentConfig.h
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+// 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 AutoplayExperimentConfig_h
+#define AutoplayExperimentConfig_h
+
+namespace WTF {
+class String;
+}
+
+namespace blink {
+
+class AutoplayExperimentConfig {
+    public:
+    // Experiment configuration bits.  These maybe combined.  For example,
+    // ForVideo|IfMuted will override the user gesture requirement for
+    // playing video that has no audio or is muted.  ForVideo, by itself,
+    // will entirely override the user gesture requirement for all video
+    // elements, but not for audio elements.
+    enum Mode {
+        // Do not enable the autoplay experiment.
+        Off        = 0,
+        // Enable gestureless autoplay for video elements.
+        ForVideo   = 1 << 0,
+        // Enable gestureless autoplay for audio elements.
+        ForAudio   = 1 << 1,
+        // Restrict gestureless autoplay to audio-less or muted media.
+        IfMuted    = 1 << 2,
+        // Restrict gestureless autoplay to sites which contain the
+        // viewport tag.
+        IfMobile   = 1 << 3,
+        // If gestureless autoplay is allowed, then mute the media before
+        // starting to play.
+        PlayMuted  = 1 << 4,
+    };
+
+    static Mode fromString(const WTF::String& token);
+};
+
+inline AutoplayExperimentConfig::Mode& operator|=(AutoplayExperimentConfig::Mode& a, const AutoplayExperimentConfig::Mode& b)
+{
+    a = static_cast<AutoplayExperimentConfig::Mode>(
+        static_cast<int>(a) | static_cast<int>(b));
+    return a;
+}
+
+} // namespace blink
+
+#endif // AutoplayExperimentConfig_h