Paint Profiler: erase bitmap before replaying SkPicture

We used to have garbage in resulting bitmap if we abort redraw before fill commands
in SkPicture are executed.

BUG=426100,426102

Review URL: https://codereview.chromium.org/654843010

git-svn-id: svn://svn.chromium.org/blink/trunk@184363 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/platform/graphics/GraphicsContextRecorder.cpp

@@ -104,6 +104,7 @@ PassOwnPtr<Vector<char> > GraphicsContextSnapshot::replay(unsigned fromStep, uns
     int height = ceil(scale * m_picture->height());
     SkBitmap bitmap;
     bitmap.allocPixels(SkImageInfo::MakeN32Premul(width, height));
+    bitmap.eraseARGB(0, 0, 0, 0);
     {
         ReplayingCanvas canvas(bitmap, fromStep, toStep);
         canvas.scale(scale, scale);