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This refactors RasterSource to make it possibly to reuse the preamble
of transform/clip/clearing that it does when moving raster to the
gpu process.  The followup to this patch will be to use this
preamble and send it as its own PaintOpBuffer ahead of the
PaintOpBuffer that gets rastered.

Bug: 671433
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ia2139f82ad0186df22afeb120fa8944625971ef9
Reviewed-on: https://chromium-review.googlesource.com/592237
Commit-Queue: enne <enne@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491487}