cc: Remove all traces of synchronous GPU rasterization

Removed threaded gpu raster flag, and syncronous gpu raster tests and
all code required for syncronous gpu rasterization.

Removed the empty software rasterizer class, and base and moved
ownership of the gpu rasterizer into the GpuTileTaskWorkerPool, since
it is still a nice separation of functionality

BUG=457860

Review URL: https://codereview.chromium.org/1063493002

Cr-Commit-Position: refs/heads/master@{#323860}
39 files changed