Bug 1362212 - Implement HTMLMediaElement.reportCanPlayTelemetry. r=gerald

Defer determining whether we have usable decoders to an off-main thread in
order to avoid janking the main thread.

MozReview-Commit-ID: Ape5zEBBMrz

dom/html/HTMLMediaElement.h

@@ -102,6 +102,8 @@ public:
 
   explicit HTMLMediaElement(already_AddRefed<mozilla::dom::NodeInfo>& aNodeInfo);
 
+  void ReportCanPlayTelemetry();
+
   /**
    * This is used when the browser is constructing a video element to play
    * a channel that we've already started loading. The src attribute and