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

Review URL: https://codereview.chromium.org/467353002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@180215 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs