Move the rest of files in child/ to common/.

The directory structure for Blink scheduler is changing. Those files are
thread-neutral utilities, so they should go to scheduler/common/.

Bug: 826203
Change-Id: I9c1fa854af7cd11c36060fc0ba8ee19ed3404a6f
Reviewed-on: https://chromium-review.googlesource.com/c/1301699
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603440}
22 files changed