Move common scheduler classes from child/ to common/.

This patch moves CancelableClosureHoler, IdleCanceledDelayedTaskSweeper
and IdleHelper from scheduler/child/ to scheduler/common/. The child/
directory is now obsolete. These classes are for both main thread and
non-main thread schedulers, so they should be used to common/.

Bug: 826203
Change-Id: I967f922084c59b191ae127209e10127c17e9bbc7
Reviewed-on: https://chromium-review.googlesource.com/1084400
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564393}
13 files changed