Remove uses of skia::GetWritablePixels(PaintCanvas)

cc/paint/paint_canvas.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_PAINT_PAINT_CANVAS_H_
 #define CC_PAINT_PAINT_CANVAS_H_
 
 #include "cc/paint/paint_export.h"
 #include "third_party/skia/include/core/SkCanvas.h"
 #include "third_party/skia/include/utils/SkNWayCanvas.h"
 #include "third_party/skia/include/utils/SkNoDrawCanvas.h"
 
 namespace cc {
 
 using PaintCanvas = SkCanvas;
 using PaintCanvasNoDraw = SkNoDrawCanvas;
 using PaintCanvasAutoRestore = SkAutoCanvasRestore;
 
 class CC_PAINT_EXPORT PaintCanvasPassThrough : public SkNWayCanvas {
  public:
   explicit PaintCanvasPassThrough(SkCanvas* canvas);
   PaintCanvasPassThrough(int width, int height);
   ~PaintCanvasPassThrough() override;
 };
 
+// PaintCanvas equivalent of skia::GetWritablePixels.
+CC_PAINT_EXPORT bool ToPixmap(PaintCanvas* canvas, SkPixmap* output);
+
 }  // namespace cc
 
 #endif  // CC_PAINT_PAINT_CANVAS_H_