Fix the rasterization resolution of zoomed canvases with display list backing

Fix the rasterization resolution of zoomed canvases with display list backing

The patch makes use of an intermediate render buffer at an alternate
resolution for rasterizing scaled canvases in a spec compliant way.

BUG=392600
TEST=layout tests with zoomed canvases (there's lots) with --enabled display-list-2d-canvas

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180215

[Justin Novosad, 2014-08-13 18:23:53]

PTAL

[Stephen White, 2014-08-13 21:26:50]

LGTM

IWBN to have some tests, but I assume we'll get that from layout tests when we
turn on display list canvas by default.

https://codereview.chromium.org/467353002/diff/20001/Source/platform/graphics...
File Source/platform/graphics/GraphicsContext.cpp (right):

https://codereview.chromium.org/467353002/diff/20001/Source/platform/graphics...
Source/platform/graphics/GraphicsContext.cpp:1124: RefPtr<SkMatrixImageFilter>
matrixFilter = adoptRef(SkMatrixImageFilter::Create(layerScale,
SkPaint::kLow_FilterLevel, pictureFilter.get()));
Perf note: using a matrix filter will incur one extra (non-scaling) blit than
the old path. I'm not sure if this will show up in benches, but it's something
to think about. Maybe we could ameliorate it by having a way in Skia for the
final filter in the DAG to take on the responsibility of the blit, instead of
targeting its own internal buffer. This would be a perf win for all filters.

[Justin Novosad, 2014-08-13 22:12:15]

The CQ bit was checked by junov@chromium.org

[commit-bot: I haz the power, 2014-08-13 22:13:03]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/junov@chromium.org/467353002/20001

[commit-bot: I haz the power, 2014-08-14 00:10:39]

Committed patchset #2 (20001) as 180215