Sequence::TakeTask() transfers ownership of the Task in the front slot of
the Sequence to the caller. This makes the front slot empty. The empty
front slot can be removed with Sequence::RemoveFrontSlot().

Benefits of transfering ownership of a Task to code that runs it:
- The code that runs (and owns) the Task can modify it. This is
  important if we want to use OnceClosure in TaskScheduler.
- It is now clear that a single thread has a pointer to a given
  Task at a time (before, multiple threads calling PeekTask on
  the same Sequence could theoretically get pointers to the same
  Task).

BUG=553459

Review-Url: https://codereview.chromium.org/2399213005
Cr-Commit-Position: refs/heads/master@{#424719}