Paint Profiler: erase bitmap before replaying SkPicture

Source/platform/graphics/GraphicsContextRecorder.cpp

Index: Source/platform/graphics/GraphicsContextRecorder.cpp
diff --git a/Source/platform/graphics/GraphicsContextRecorder.cpp b/Source/platform/graphics/GraphicsContextRecorder.cpp
index 8aa6d524f4335456f52371267fac738b0ee14a6b..3b2b6d76d9ba3bc7c4bfd2e8254f3f10a23fe085 100644
--- a/Source/platform/graphics/GraphicsContextRecorder.cpp
+++ b/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);