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 * Actually running the tasks in the ordered you asked!
 * Allow running only pending tasks, all tasks until idle, to a given time or
   for a given period.
 * Allow stopping of running tasks on *any* arbitrary condition. No longer will
   your tasks stop working when someone adds a new task or changes the task
   order!
 * Task runner intimately connected to time and controls Now().
   Supports both automatic management and manual control.

This change makes it possible for the scheduler_unit tests to be 100%
deterministic. It also allows them to be more flexible and less brittle.

BUG=380889

Review URL: https://codereview.chromium.org/387493002

Cr-Commit-Position: refs/heads/master@{#294059}