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This change causes RasterWorkerPool to handle task
categories. To do this RasterWorkerPool assigns
specific task categories to each of its threads.

As I've been having trouble showing a clear win/loss
for using a single thread for background tasks, this
is behind a flag
"--use-single-thread-for-background-raster-tasks",
allowing us to test this more.

BUG=564260
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel
CQ_EXTRA_TRYBOTS=tryserver.chromium.perf:linux_perf_bisect;tryserver.chromium.perf:mac_10_10_perf_bisect;tryserver.chromium.perf:win_perf_bisect

Review URL: https://codereview.chromium.org/1576133002

Cr-Commit-Position: refs/heads/master@{#370812}