[MessageLoopCurrent] Move QuitWhenIdleClosure to RunLoop

I was about to migrate MessageLoop::QuitWhenIdleClosure() to
MessageLoopCurrent::QuitWhenIdleClosure() but figured this should really
just be alongside similar deprecated methods moved to RunLoop last year.

This CL is a contract that I'll TBR mass migrate calls to
MessageLoop::QuitWhenIdleClosure() to it after this lands
(and fix includes where relevant).

[email protected], [email protected]

Bug: 825327
Change-Id: I3a544a3735427dfac5e57c4e266be9901fbb67ec
Reviewed-on: https://chromium-review.googlesource.com/1055808
Reviewed-by: kylechar <[email protected]>
Reviewed-by: danakj <[email protected]>
Commit-Queue: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/master@{#558956}
2 files changed