Implement MediaStreamTrack contentHint skeleton.

third_party/WebKit/Source/modules/mediastream/MediaStreamTrack.idl

Index: third_party/WebKit/Source/modules/mediastream/MediaStreamTrack.idl
diff --git a/third_party/WebKit/Source/modules/mediastream/MediaStreamTrack.idl b/third_party/WebKit/Source/modules/mediastream/MediaStreamTrack.idl
index f96f1b2cdc51d375ecf99a80669b2bc820f8539d..335adfd82461aef8ad58baac633659e84d0aa311 100644
--- a/third_party/WebKit/Source/modules/mediastream/MediaStreamTrack.idl
+++ b/third_party/WebKit/Source/modules/mediastream/MediaStreamTrack.idl
@@ -42,6 +42,7 @@ enum MediaStreamTrackState {
     readonly attribute DOMString label;
     attribute boolean enabled;
     readonly attribute boolean muted;
+    [RuntimeEnabled=MediaStreamTrackContentHint] attribute DOMString contentHint;
     attribute EventHandler onmute;
     attribute EventHandler onunmute;
     readonly attribute MediaStreamTrackState readyState;
@@ -52,6 +53,7 @@ enum MediaStreamTrackState {
     [RuntimeEnabled=MediaConstraints] MediaTrackConstraints getConstraints();
     [RuntimeEnabled=MediaGetSettings] MediaTrackSettings getSettings();
 
+
     // Non-standard APIs
     [MeasureAs=MediaStreamTrackRemote] readonly attribute boolean remote;
 };