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With this CL, tasks posted to the base/task_scheduler/post_task.h API
no longer run asynchronously on a dedicated thread. Instead, they are
posted to a MessageLoop on the thread where the ScopedTaskScheduler
lives and run synchronously when RunLoop::Run/RunUntilIdle() is
invoked. This CL also allows usage of the
base/task_scheduler/post_task.h API within the scope of a
TestBrowserThreadBundle.

Benefits of running TaskScheduler tasks synchronously:
- Tests are more deterministic.
- It is easier to wait for a chain of tasks involving TaskScheduler.
  E.g.:
   base::PostTaskAndReplyWithTraits(
       FROM_HERE, TaskTraits(), Bind(&A), Bind(&B));
   base::RunLoop().RunUntilIdle();
       // Without this CL, this returns immediately because there
       // are no pending tasks on the main thread initially. With
       // this CL, this returns when A and B have run.

Note that this CL prevents a TestBrowserThreadBundle or a MessageLoop
from being initialized in the scope of a ScopedTaskScheduler. It
fixes tests that previously did this.

BUG=553459

Review-Url: https://codereview.chromium.org/2557083002
Cr-Commit-Position: refs/heads/master@{#439505}