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/HTMLMediaElement.cpp

@@ -7153,6 +7153,17 @@ HTMLMediaElement::NextFrameStatus()
   return NEXT_FRAME_UNINITIALIZED;
 }
 
+
+void
+HTMLMediaElement::SetDecoder(MediaDecoder* aDecoder)
+{
+  MOZ_ASSERT(aDecoder); // Use ShutdownDecoder() to clear.
+  if (mDecoder) {
+    ShutdownDecoder();
+  }
+  mDecoder = aDecoder;
+}
+
 float
 HTMLMediaElement::ComputedVolume() const
 {