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 daacc3017889050f52f4facfe35ae8b0053ba22b..199d9d92ed8dc72e883461f00a47e4deccd0fb81 100644
--- a/cc/paint/paint_canvas.h
+++ b/cc/paint/paint_canvas.h
@@ -10,19 +10,24 @@
 #include "base/memory/ref_counted.h"
 #include "build/build_config.h"
 #include "cc/paint/paint_export.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
@@ -41,7 +46,7 @@ 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;
@@ -92,6 +97,8 @@ 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,