cc: Make PaintCanvas abstract

cc/paint/paint_recorder.h

Index: cc/paint/paint_recorder.h
diff --git a/cc/paint/paint_recorder.h b/cc/paint/paint_recorder.h
index 752dadd1a0d2b5b52b9c0691581668c84919cd4b..b48b906ca35a5bca9192346102c3faabf1fcf0ae 100644
--- a/cc/paint/paint_recorder.h
+++ b/cc/paint/paint_recorder.h
@@ -11,6 +11,7 @@
 #include "base/optional.h"
 #include "cc/paint/paint_canvas.h"
 #include "cc/paint/paint_record.h"
+#include "cc/paint/skia_paint_canvas.h"
 #include "third_party/skia/include/core/SkPictureRecorder.h"
 
 namespace cc {
@@ -61,7 +62,7 @@ class CC_PAINT_EXPORT PaintRecorder {
 
  private:
   SkPictureRecorder recorder_;
-  base::Optional<PaintCanvas> canvas_;
+  base::Optional<SkiaPaintCanvas> canvas_;
 
   DISALLOW_COPY_AND_ASSIGN(PaintRecorder);
 };