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This adds microbenchmarks to measure the cost of posting a task to
a message loop running different pump types. This measures the wall and
(where supported) thread time that elapses from different numbers of
threads posting to one target. This is not designed to measure the time
taken by the target thread or fairness of posting threads.

Each test is set up with one target thread and 1-4 posting threads. The
posting threads each run one task that posts batches of tasks to the
target thread until they have been working for at least 5 seconds of
wall time. The tasks on the target thread simply increment a counter.
The test runner starts each posting thread, posts a start task to each
then joins them and then joins the target thread and aggregates stats.

BUG=412137

Review URL: https://codereview.chromium.org/551183002

Cr-Commit-Position: refs/heads/master@{#297264}