Add 'autoplay' content settings and expose it to Blink.

third_party/WebKit/public/web/WebContentSettingsClient.h

Index: third_party/WebKit/public/web/WebContentSettingsClient.h
diff --git a/third_party/WebKit/public/web/WebContentSettingsClient.h b/third_party/WebKit/public/web/WebContentSettingsClient.h
index eaa4368bd039600da45e1417337d0d0cca9e9def..b83afd027c7c074d6f8b5aecee2d3a4ae2e21834 100644
--- a/third_party/WebKit/public/web/WebContentSettingsClient.h
+++ b/third_party/WebKit/public/web/WebContentSettingsClient.h
@@ -78,6 +78,9 @@ public:
     // but it's been named for consistency with the rest of the interface.
     virtual bool allowMutationEvents(bool defaultValue) { return defaultValue; }
 
+    // Controls whether autoplay is allowed for this frame.
+    virtual bool allowAutoplay(bool defaultValue) { return defaultValue; }
+
     // Notifies the client that the frame would have instantiated a plugin if plugins were enabled.
     virtual void didNotAllowPlugins() { }