Migrate base::TaskRunner from Closure to OnceClosure

After this CL, TaskRunner::PostTask and its family can take OnceClosure
in addition to Closure.

Most of the changes are mechanical replacement of Closure with OnceClosure
on TaskRunner family. Others are:
 - Limit CriticalClosure from Closure to OnceClosure as no caller call
   the resulting callback more than once
 - Add several PostTaskAndReplyWithResult overloads for old Callback
   version, for compatibility. (in base/task_scheduler/post_task.h)
 - Update SequencedWorkerPool implementation for OnceClosure.
 - Update task handling code in app_state.mm for OnceClosure, which is
   needed to bring OnceClosure into a ObjC block.


Review-Url: https://codereview.chromium.org/2637843002
Cr-Commit-Position: refs/heads/master@{#462023}
110 files changed