Include viewport visibility checks for autoplay 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
index e1dd6daa4ca8ae2d216aa7a2eb938f05be704f31..7392a6acf1b6cb2b547d5e3e65c65a448adf76f8 100644
--- a/Source/core/html/AutoplayExperimentConfig.h
+++ b/Source/core/html/AutoplayExperimentConfig.h
@@ -14,8 +14,8 @@ 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,
+    // ForVideo|IfViewport will override the user gesture requirement for
+    // playing video that is visible in the viewport.  ForVideo, by itself,
     // will entirely override the user gesture requirement for all video
     // elements, but not for audio elements.
     enum Mode {
@@ -25,14 +25,17 @@ class AutoplayExperimentConfig {
         ForVideo   = 1 << 0,
         // Enable gestureless autoplay for audio elements.
         ForAudio   = 1 << 1,
+        // Restrict gestureless autoplay to media that is visible in
+        // the viewport.
+        IfViewport = 1 << 2,

[philipj_slow, 2015/09/04 08:47:56]

Ah, here it is. Here's where a document.visibility-only experiment would be
useful.

         // Restrict gestureless autoplay to audio-less or muted media.
-        IfMuted    = 1 << 2,
+        IfMuted    = 1 << 3,
         // Restrict gestureless autoplay to sites which contain the
         // viewport tag.
-        IfMobile   = 1 << 3,
+        IfMobile   = 1 << 4,
         // If gestureless autoplay is allowed, then mute the media before
         // starting to play.
-        PlayMuted  = 1 << 4,
+        PlayMuted  = 1 << 5,
     };
 
     static Mode fromString(const WTF::String& token);