Add options for audio/video autoplay to chrome://settings/content.

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 dc957e1f5e4cde1cf2e9410ca3ec9c45f9033698..3ed3e4b699e98473f1b7b891870c70c47229e952 100644
--- a/third_party/WebKit/public/web/WebContentSettingsClient.h
+++ b/third_party/WebKit/public/web/WebContentSettingsClient.h
@@ -33,6 +33,9 @@ public:
     // Controls whether HTML5 media elements (<audio>, <video>) are allowed for this frame.
     virtual bool allowMedia(const WebURL& videoURL) { return true; }
 
+    // Controls whether HTML5 media elements (<audio>, <video>) are allowed to autoplay content.
+    virtual bool allowMediaAutoplay(bool hidden) { return true; }

[philipj_slow, 2015/10/23 12:17:30]

AFAICT, the hidden bool comes only from HTMLMediaElement, and it is always
false. What is it actually intended to do? I suspect that it would be cleaner to
expose all three states to HTMLMediaElement and decide there whether or not to
autoplay. The code as it is de-facto only has two states of course, so unless
something is added to the CL you might want to simplify and add the extra state
only when it's properly supported.

[DaleCurtis, 2015/10/23 19:21:27]

This is called by the Chrome content client with the actual hidden state, but
the FrameLoader APIs could have the hidden state removed. 

[philipj_slow, 2015/10/28 10:30:40]

Could we instead use mediaElement.document().hidden()? For web-observable
behavior it's really nice if the behavior can be expressed in terms of other
web-exposed APIs, so that web developers can see the same state and understand
what is happening.

+
     // Controls whether plugins are allowed for this frame.
     virtual bool allowPlugins(bool enabledPerSettings) { return enabledPerSettings; }