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This patch adds the ability to suspend and resume the context, which
when suspended prevents future tasks from running.

As of right now, the work isn't prevented from finishing since we don't
have co-operative work implemented and nothing is preventing the rest
of the system from processing the changes.

Added tests, a few of which fail due to above reason.

R=chrishtr@chromium.org

Bug: 882633
Change-Id: I256be53b3cd0071405c9ac783fe78fcd36dacb9c
Reviewed-on: https://chromium-review.googlesource.com/c/1289432Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601219}