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These histograms gave an approximate idea of the latency between when a
task is posted and when the ThreadPool starts executing it. However,
they were imperfect:

- If the task is posted just before the system sleeps, it may run
  after the system wakes up from sleep, resulting in a long latency
  being reported.
- The task is more likely to be posted just after the system wakes
  up from sleep.
- The task can be posted in a background renderer. The latency will
  be high because the process priority is low. We have no way to
  distinguish this case.
- The task can be posted when the user is not actively using
  Chrome. The latency will be low because there are no other tasks.
  We have no way to distinguish this case.

Fixed: 1089861
Change-Id: Ibc4c8dc6c4e2ce38e69e7ccfd9ddcda0218dd20a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526173
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831910}