Back PaintRecord with PaintOpBuffer instead of SkPicture

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 6b9a0df08b9358715bde09dd51f7aef88d7d0775..d45e84bc0586e33ec65e25897bec4754a0917983 100644
--- a/cc/paint/paint_canvas.h
+++ b/cc/paint/paint_canvas.h
@@ -11,19 +11,24 @@
 #include "build/build_config.h"
 #include "cc/paint/paint_export.h"
 #include "cc/paint/paint_image.h"
-#include "cc/paint/paint_record.h"
 #include "third_party/skia/include/core/SkCanvas.h"
 
 namespace cc {
 
 class DisplayItemList;
 class PaintFlags;
+class PaintOpBuffer;
+
+using PaintRecord = PaintOpBuffer;
 
 class CC_PAINT_EXPORT PaintCanvas {
  public:
   virtual ~PaintCanvas() {}
 
   virtual SkMetaData& getMetaData() = 0;
+
+  // TODO(enne): this only appears to mostly be used to determine if this is
+  // recording or not, so could be simplified or removed.
   virtual SkImageInfo imageInfo() const = 0;
 
   // TODO(enne): It would be nice to get rid of flush() entirely, as it
@@ -42,7 +47,7 @@ class CC_PAINT_EXPORT PaintCanvas {
                            int y) = 0;
   virtual int save() = 0;
   virtual int saveLayer(const SkRect* bounds, const PaintFlags* flags) = 0;
-  virtual int saveLayerAlpha(const SkRect* bounds, U8CPU alpha) = 0;
+  virtual int saveLayerAlpha(const SkRect* bounds, uint8_t alpha) = 0;
 
   virtual void restore() = 0;
   virtual int getSaveCount() const = 0;
@@ -93,6 +98,8 @@ class CC_PAINT_EXPORT PaintCanvas {
   virtual bool getDeviceClipBounds(SkIRect* bounds) const = 0;
   virtual void drawColor(SkColor color, SkBlendMode mode) = 0;
   void drawColor(SkColor color) { drawColor(color, SkBlendMode::kSrcOver); }
+
+  // TODO(enne): This is a synonym for drawColor with kSrc.  Remove it.
   virtual void clear(SkColor color) = 0;
 
   virtual void drawLine(SkScalar x0,