These histograms:
- TaskScheduler.TaskLatency.BackgroundPool.BackgroundTaskPriority
- TaskScheduler.TaskLatency.BackgroundPool.UserVisibleTaskPriority
- TaskScheduler.TaskLatency.BackgroundPool.UserBlockingTaskPriority
- TaskScheduler.TaskLatency.BackgroundFileIOPool.BackgroundTaskPriority
- TaskScheduler.TaskLatency.BackgroundFileIOPool.UserVisibleTaskPriority
- TaskScheduler.TaskLatency.BackgroundFileIOPool.UserBlockingTaskPriority
- TaskScheduler.TaskLatency.ForegroundPool.BackgroundTaskPriority
- TaskScheduler.TaskLatency.ForegroundPool.UserVisibleTaskPriority
- TaskScheduler.TaskLatency.ForegroundPool.UserBlockingTaskPriority
- TaskScheduler.TaskLatency.ForegroundFileIOPool.BackgroundTaskPriority
- TaskScheduler.TaskLatency.ForegroundFileIOPool.UserVisibleTaskPriority
- TaskScheduler.TaskLatency.ForegroundFileIOPool.UserBlockingTaskPriority
are replaced with:
- TaskScheduler.TaskLatency.BackgroundTaskPriority
- TaskScheduler.TaskLatency.BackgroundTaskPriority.MayBlock
- TaskScheduler.TaskLatency.UserVisibleTaskPriority
- TaskScheduler.TaskLatency.UserVisibleTaskPriority.MayBlock
- TaskScheduler.TaskLatency.UserBlockingTaskPriority
- TaskScheduler.TaskLatency.UserBlockingTaskPriority.MayBlock

Not having a pool name in task latency histogram names will allow
us to keep track of the latency of each type of task when running
experiments that change the way TaskTraits are mapped to pools.

BUG=553459

Review-Url: https://codereview.chromium.org/2600023002
Cr-Commit-Position: refs/heads/master@{#442022}