cc: Make PaintCanvas abstract

cc/paint/paint_surface.h

Index: cc/paint/paint_surface.h
diff --git a/cc/paint/paint_surface.h b/cc/paint/paint_surface.h
index 471901732aab3fb200855256587ee0b1c9327fb1..5e56ed1f94c8ec212a0444a1812881ff6fcf4941 100644
--- a/cc/paint/paint_surface.h
+++ b/cc/paint/paint_surface.h
@@ -8,8 +8,8 @@
 #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 {
@@ -49,7 +49,7 @@ class CC_PAINT_EXPORT PaintSurface : public SkRefCntBase {
 
  private:
   const sk_sp<SkSurface> surface_;
-  base::Optional<PaintCanvas> canvas_;
+  base::Optional<SkiaPaintCanvas> canvas_;
 
   DISALLOW_COPY_AND_ASSIGN(PaintSurface);
 };