Clear SkBitmap used to draw DragImage.

Source/platform/DragImage.cpp

 /*
  * Copyright (C) 2007 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 31 matching lines @@
 #include "platform/graphics/paint/DisplayItemList.h"
 #include "platform/graphics/paint/DrawingRecorder.h"
 #include "platform/graphics/skia/NativeImageSkia.h"
 #include "platform/text/BidiTextRun.h"
 #include "platform/text/StringTruncator.h"
 #include "platform/text/TextRun.h"
 #include "platform/transforms/AffineTransform.h"
 #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"
 #include "wtf/text/WTFString.h"
 
 #include <algorithm>
 
 namespace blink {
 
 const float kDragLabelBorderX = 4;
@@ skipping 27 matching lines @@
         if (orientation != DefaultImageOrientation) {
             FloatRect destRect(FloatPoint(), sizeRespectingOrientation);
             if (orientation.usesWidthAsHeight())
                 destRect = destRect.transposedRect();
 
             SkBitmap skBitmap;
             if (!skBitmap.tryAllocN32Pixels(sizeRespectingOrientation.width(), sizeRespectingOrientation.height()))
                 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);
 
             return adoptPtr(new DragImage(skBitmap, deviceScaleFactor));
         }
     }
 
     SkBitmap skBitmap;
     if (!bitmap->bitmap().copyTo(&skBitmap, kN32_SkColorType))
         return nullptr;
@@ skipping 176 matching lines @@
             *pixel = SkPreMultiplyARGB(
                 SkColorGetA(*pixel) * fraction,
                 SkColorGetR(*pixel),
                 SkColorGetG(*pixel),
                 SkColorGetB(*pixel));
         }
     }
 }
 
 } // namespace blink