cc: Use worker context for one-copy tile initialization.

This moves management of staging resources to
OneCopyTileTaskWorkerPool class. This makes it possible
to use a worker context to issue and detect when copy
operations complete.

BUG=490295
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1230203007

Cr-Commit-Position: refs/heads/master@{#342235}
55 files changed