Issue #2073 - m-c 1383404: make SourceBuffer::Compact more efficient (squashed)

The first part also means that Compact no longer needs the SurfaceCache lock (used to be via CreateChunk->CanHold),
which avoids potential deadlocks during shutdown that m-c 523950 would otherwise cause

image/ImageFactory.h

@@ -51,9 +51,10 @@ public:
    * the usual image loading mechanism.
    *
    * @param aMimeType      The mimetype of the image.
+   * @param aSizeHint      The length of the source data for the image.
    */
   static already_AddRefed<Image>
-  CreateAnonymousImage(const nsCString& aMimeType);
+  CreateAnonymousImage(const nsCString& aMimeType, uint32_t aSizeHint = 0);
 
   /**
    * Creates a new multipart/x-mixed-replace image wrapper, and initializes it