cc: Include pictures in trace output for DisplayItemLists

cc/resources/drawing_display_item.cc

 // Copyright 2014 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.
 
 #include "cc/resources/drawing_display_item.h"
 
 #include "third_party/skia/include/core/SkCanvas.h"
 #include "third_party/skia/include/core/SkDrawPictureCallback.h"
 #include "third_party/skia/include/core/SkMatrix.h"
 #include "third_party/skia/include/core/SkPicture.h"
@@ skipping 13 matching lines @@
                                 SkDrawPictureCallback* callback) const {
   canvas->save();
   canvas->translate(location_.x(), location_.y());
   if (callback)
     picture_->playback(canvas, callback);
   else
     canvas->drawPicture(picture_.get());
   canvas->restore();
 }
 
+void DrawingDisplayItem::RasterForTracing(SkCanvas* canvas) const {

[vmpstr, 2015/01/08 21:44:34]

This seems to be basically ::Raster with nullptr as callback (except it would
use drawPicture instead of playback). 

Is there a reason this has to be a different function?

[ajuma, 2015/01/08 22:20:58]

We need playback instead of drawPicture to make the tracing output useful. If we
use drawPicture, the resulting SkPicture is just a sequence of drawPicture ops,
one for each DrawingDisplayItem. The picture debugger in about:tracing doesn't
go deeper for drawPicture ops, so all we see is a sequence of drawPicture ops
and not much else. With playback, the SkPicture consists of the individual ops. 

[vmpstr, 2015/01/08 22:45:59]

Ah I see. FWIW, you can just Raster and give is some small class DontAbort :
public SkDrawPictureCallback { bool abortDrawing() override { return false; } };
type of thing. However, this works as well.

Either way, can you make a comment here that explains why this should be
different (or why we need to pass a don't abort callback if you go that route)?

[ajuma, 2015/01/08 23:16:11]

Added a comment.

+  canvas->save();
+  canvas->translate(location_.x(), location_.y());
+  picture_->playback(canvas);
+  canvas->restore();
+}
+
 bool DrawingDisplayItem::IsSuitableForGpuRasterization() const {
   return picture_->suitableForGpuRasterization(NULL);
 }
 
 int DrawingDisplayItem::ApproximateOpCount() const {
   return picture_->approximateOpCount() + sizeof(gfx::PointF);
 }
 
 size_t DrawingDisplayItem::PictureMemoryUsage() const {
   DCHECK(picture_);
   return SkPictureUtils::ApproximateBytesUsed(picture_.get());
 }
 
 }  // namespace cc