cc: Discard SkCanvas (frame-buffer) before rasterization.

cc/resources/picture_pile_impl.cc

Index: cc/resources/picture_pile_impl.cc
diff --git a/cc/resources/picture_pile_impl.cc b/cc/resources/picture_pile_impl.cc
index 3f8e167b3c5d067e2531401ba21da2698691604c..8caeb687846834dce7eaca9fbadd132f9e377186 100644
--- a/cc/resources/picture_pile_impl.cc
+++ b/cc/resources/picture_pile_impl.cc
@@ -97,6 +97,9 @@ void PicturePileImpl::RasterToBitmap(
     const gfx::Rect& canvas_rect,
     float contents_scale,
     RenderingStatsInstrumentation* rendering_stats_instrumentation) {
+  DCHECK(canvas->getBaseLayerSize().equals(canvas_rect.width(),
+                                           canvas_rect.height()));

[reveman, 2014/05/09 00:56:29]

Should we really require this? In that case, should |canvas_rect| be
|canvas_origin| instead of passing more data to this function than needed?

[alokp, 2014/05/09 20:13:04]

We do not require it as far as the API is concerned. But in practice we always
create a new canvas for each raster. The API allows you to offset multiple
rasters into a single canvas. But we have gradually started making the
assumption that it is OK to write outside canvas_rect - see line 143 below where
we clear the entire canvas. I wanted to add a DCHECK to ensure this assumption.

You are right that we should change the API to make this assumption explicit.
But I think it is outside the scope of this patch.

[reveman, 2014/05/10 00:34:52]

I think I prefer if you didn't add this DCHECK. Without it, this API can be
interpreted as "Here's a canvas of some size, you can do whatever with it but
canvas_rect is the area I like correct contents to be produced for". If this
flexibility is unnecessary, I'd rather see us change the API right away than add
temporary DCHECKs.

[alokp, 2014/05/12 18:45:33]

OK. Removed DCHECK. I think we should make it clear through documentation or
otherwise that this function may write over area outside canvas_rect.

[reveman, 2014/05/12 18:51:38]

Yea, that wouldn't hurt. Feel free to add a comment to picture_pile_impl.h as
part of this patch if you like.

+  canvas->discard();
   if (clear_canvas_with_debug_color_) {
     // Any non-painted areas will be left in this color.
     canvas->clear(DebugColors::NonPaintedFillColor());