[base] Make Sequence/ThreadChecker move-construct/assignable

This will make it possible to define a default move-assign/construct
on classes that use a THREAD/SEQUENCE_CHECKER..

e.g. https://chromium-review.googlesource.com/c/chromium/src/+/1693302

R=fdoray@chromium.org

Bug: 946657
Change-Id: Ie2b40984b664fa1dcdcdd093ab546625db685430
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1696845
Commit-Queue: Gabriel Charette <gab@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682436}