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These new tests measure performance of dispatching incoming messages on the same
thread.

Currently on Linux:
RouterDispatchCost  3.93483e+06 times/second
MultiplexRouterDispatchCost 3.39754e+06 times/second

BUG=594244

Review-Url: https://codereview.chromium.org/2340363002
Cr-Commit-Position: refs/heads/master@{#419007}