cc: Add BitmapRasterWorkerPool.
This adds a rasterizer implementation explicitly used for
rasterizing to bitmap resources. Each rasterizer is already
aware of specific resource details and it is much easier
and less confusing to implement a different rasterizer that
handle bitmap resources then trying to emulate support for
GL-ish APIs that are used by other rasterizer implementations.
This allows us to clean up LTHI a bit and reduces the
complexity of the ResourceProvider class significantly.
It also makes future changes that will allow GpuMemoryBuffers
to be created and mapped in worker threads easier implement.
BUG=
Review URL: https://codereview.chromium.org/
602493003
Cr-Commit-Position: refs/heads/master@{#296558}