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Instead of relying on sleeping and the actual clock, use a mock
taskrunner to simulate the passage of time. Makes the test more accurate
and stable, and it also speeds things up significantly (~1.5s->60-70ms).

Also convert ProcTask to use unique_ptr and WeakPtrs instead of ref
counting to more cleanly cleanup the memory on completion instead of
waiting for unused scheduled retries.

Change-Id: I22b741d8b29d8eb419d125147ef98eba01c658e4
Reviewed-on: https://chromium-review.googlesource.com/1108078Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570840}