Clear SkBitmap used to draw DragImage.

Source/platform/DragImage.cpp

Index: Source/platform/DragImage.cpp
diff --git a/Source/platform/DragImage.cpp b/Source/platform/DragImage.cpp
index ce770dbd25e654a15112fdd5d7351495186c4da2..cf110a3763856ebbeb334a80b7887854d86bb300 100644
--- a/Source/platform/DragImage.cpp
+++ b/Source/platform/DragImage.cpp
@@ -49,6 +49,7 @@
 #include "platform/weborigin/KURL.h"
 #include "skia/ext/image_operations.h"
 #include "third_party/skia/include/core/SkCanvas.h"
+#include "third_party/skia/include/core/SkColor.h"
 #include "third_party/skia/include/core/SkMatrix.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/RefPtr.h"
@@ -96,6 +97,7 @@ PassOwnPtr<DragImage> DragImage::create(Image* image, RespectImageOrientationEnu
                 return nullptr;
 
             SkCanvas canvas(skBitmap);
+            canvas.clear(SK_ColorTRANSPARENT);

[f(malita), 2015/02/05 18:02:33]

Nit: this is technically subject to the (default) clip, so just for clarity I
would call skBitmap.eraseColor() instead.

[jbroman, 2015/02/05 18:07:57]

Done. I was waffling between the two, and I'm perfectly happy eraseColor.

             canvas.concat(affineTransformToSkMatrix(orientation.transformFromDefault(sizeRespectingOrientation)));
             canvas.drawBitmapRect(bitmap->bitmap(), 0, destRect);