Revert 255322 "ThreadWatcher: fixes Start/StopWatchingAll."

> ThreadWatcher: fixes Start/StopWatchingAll.
> 
> Start is deferred in relation to Stop, and when they're both called
> within the Start interval, the end result was started.
> 
> BUG=347887
> TEST=unit_tests --gtest_filter=ThreadWatcherList*
> 
> Review URL: https://codereview.chromium.org/183063007

BUG=347887,349987
TBR=bulach@chromium.org

Review URL: https://codereview.chromium.org/188643006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255401 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255401