Add SW rendering flavor to CanvasResourceGpuMemoryBuffer

This change adds a software rendering mode to
CanvasResourceGpuMemoryBuffer.  This is meant to replace (in a
follow-up CL) the presentation code path for software rendered 2d
canvases.

BUG=788439

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I8656e4efd82e1d1e6fbb853dd7dc235fd72960e0
Reviewed-on: https://chromium-review.googlesource.com/1083146
Commit-Queue: Justin Novosad <junov@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564572}
6 files changed