Bug 1234744 - Fix build errors in ImageBitmap.h. r=roc

--HG--
extra : rebase_source : 6832c025216fd26cdd5932dc1a789b5080621380

dom/canvas/ImageBitmap.h

@@ -26,6 +26,7 @@ class ErrorResult;
 
 namespace gfx {
 class DataSourceSurface;
+class DrawTarget;
 class SourceSurface;
 }