Bug 1390748 - Pre-declare MediaDecoder in HTMLMediaElement. r=jwwang

I noticed that touching MediaDecoder rebuilds a lot of seemingly unrelated
code. This is because HTMLMediaElement includes MediaDecoder.h, and
HTMLMediaElement is included in a number of places. Having HTMLMediaElement.h
predeclare rather than include fixes it.

MozReview-Commit-ID: I0vrPgqvvge

dom/html/HTMLVideoElement.h

@@ -11,6 +11,9 @@
 #include "mozilla/dom/HTMLMediaElement.h"
 
 namespace mozilla {
+
+class FrameStatistics;
+
 namespace dom {
 
 class WakeLock;