TaskScheduler [13] TaskSchedulerImpl

TaskSchedulerImpl is the default implementation of the TaskScheduler
interface. It runs tasks in 4 thread pools:
- Background with file i/o
- Background without file i/o
- Normal with file i/o
- Normal without file i/o

This change completes the necessary changes described in
https://codereview.chromium.org/1698183005/

BUG=553459

Review-Url: https://codereview.chromium.org/1701343003
Cr-Commit-Position: refs/heads/master@{#392063}
8 files changed