Issue #80 - De-unify dom/media part 1 (updated reland)

This de-unifies up to and including dom/media/platforms with the exception of
dom/media/platforms/wmf, because that one throws weird errors in the Win
SDK which don't have a clear cause (yet)

dom/media/platforms/agnostic/gmp/GMPVideoDecoder.cpp

@@ -10,6 +10,7 @@
 #include "prsystem.h"
 #include "MediaData.h"
 #include "GMPDecoderModule.h"
+#include "MP4Decoder.h"
 #include "VPXDecoder.h"
 
 namespace mozilla {