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: svn://svn.chromium.org/blink/trunk@180215 bbb929c8-8fbe-4397-9dbb-9b2b20218538