cc: Make PaintCanvas abstract

cc/paint/paint_surface.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_SURFACE_H_
 #define CC_PAINT_PAINT_SURFACE_H_
 
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/optional.h"
-#include "cc/paint/paint_canvas.h"
 #include "cc/paint/paint_export.h"
+#include "cc/paint/skia_paint_canvas.h"
 #include "third_party/skia/include/core/SkSurface.h"
 
 namespace cc {
 
 class CC_PAINT_EXPORT PaintSurface : public SkRefCntBase {
  public:
   explicit PaintSurface(sk_sp<SkSurface> surface);
   ~PaintSurface() override;
 
   // TODO(enne): this interface matches SkSurface for simplicity.
@@ skipping 19 matching lines @@
   uint32_t generationID() { return surface_->generationID(); }
 
   PaintCanvas* getCanvas() {
     if (!canvas_.has_value())
       canvas_.emplace(surface_->getCanvas());
     return &canvas_.value();
   }
 
  private:
   const sk_sp<SkSurface> surface_;
-  base::Optional<PaintCanvas> canvas_;
+  base::Optional<SkiaPaintCanvas> canvas_;
 
   DISALLOW_COPY_AND_ASSIGN(PaintSurface);
 };
 
 }  // namespace cc
 
 #endif  // CC_PAINT_PAINT_SURFACE_H_