GitLab

In order to more easily refactor RasterSource's preamble generation
(see https://chromium-review.googlesource.com/c/592237/) this moves
out code that isn't supported by PaintCanvas up to where there's
an SkCanvas.

The SkCanvas::discard logic only applies to gpu raster and so is
only replicated in the gpu raster buffer provider and not elsewhere.

Bug: 671433
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I0e006b78bb619b56e5db0a1d30c74e8de63547e1
Reviewed-on: https://chromium-review.googlesource.com/594661
Commit-Queue: enne <enne@chromium.org>
Reviewed-by: Vladimir Levin <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491202}