Add TaskScheduler.NumTasksBetweenWaits.[pool] histogram.

This histogram gives the number of tasks executed by a
SchedulerWorker between two waits on its WaitableEvent.

BUG=553459

Review-Url: https://codereview.chromium.org/2362743002
Cr-Commit-Position: refs/heads/master@{#421324}


CrOS-Libchrome-Original-Commit: 628580f4cb1d4bf9c834ee9fd203b9c69def1417
3 files changed
tree: 19f689bacc7bb78999e89294eb6874cb79627610
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/