Issue #2357 - WebM demuxer can't tell when a video wants to be transparent.

Required to have an alpha channel (which in the strange world of graphics seems to refer to transparency, and not experimental code) in WebM videos. Straight port of Firefox 53 implementation, adapted slightly for a couple of patches previously taken without this.

Ref: BZ 1320829

dom/media/MediaData.h

@@ -622,15 +622,22 @@ private:
 class MediaRawData : public MediaData {
 public:
   MediaRawData();
-  MediaRawData(const uint8_t* aData, size_t mSize);
+  MediaRawData(const uint8_t* aData, size_t aSize);
+  MediaRawData(const uint8_t* aData, size_t aSize,
+               const uint8_t* aAlphaData, size_t aAlphaSize);
 
   // Pointer to data or null if not-yet allocated
   const uint8_t* Data() const { return mBuffer.Data(); }
+  // Pointer to alpha data or null if not-yet allocated
+  const uint8_t* AlphaData() const { return mAlphaBuffer.Data(); }
   // Size of buffer.
   size_t Size() const { return mBuffer.Length(); }
+  size_t AlphaSize() const { return mAlphaBuffer.Length(); }
   size_t ComputedSizeOfIncludingThis() const
   {
-    return sizeof(*this) + mBuffer.ComputedSizeOfExcludingThis();
+    return sizeof(*this)
+           + mBuffer.ComputedSizeOfExcludingThis()
+           + mAlphaBuffer.ComputedSizeOfExcludingThis();
   }
   // Access the buffer as a Span.
   operator Span<const uint8_t>() { return MakeSpan(Data(), Size()); }
@@ -661,6 +668,7 @@ protected:
 private:
   friend class MediaRawDataWriter;
   AlignedByteBuffer mBuffer;
+  AlignedByteBuffer mAlphaBuffer;
   CryptoSample mCryptoInternal;
   MediaRawData(const MediaRawData&); // Not implemented
 };