Remove uses of skia::GetWritablePixels(PaintCanvas)

cc/paint/paint_canvas.h

Index: cc/paint/paint_canvas.h
diff --git a/cc/paint/paint_canvas.h b/cc/paint/paint_canvas.h
index c0d8bf3a11ed142c46b56434042ee64ea31d0587..6df34439c315f69544fe83ae0d27c7be7815e456 100644
--- a/cc/paint/paint_canvas.h
+++ b/cc/paint/paint_canvas.h
@@ -23,6 +23,9 @@ class CC_PAINT_EXPORT PaintCanvasPassThrough : public SkNWayCanvas {
   ~PaintCanvasPassThrough() override;
 };
 
+// PaintCanvas equivalent of skia::GetWritablePixels.
+CC_PAINT_EXPORT bool ToPixmap(PaintCanvas* canvas, SkPixmap* output);
+
 }  // namespace cc
 
 #endif  // CC_PAINT_PAINT_CANVAS_H_