Revert 228889 "Avoid threading races on TraceSamplingThread's me..."

> Avoid threading races on TraceSamplingThread's members.
> 
> See a TSAN log in the bug for more details.
> 
> BUG=305459
> 
> Review URL: https://codereview.chromium.org/26541005

TBR=haraken@chromium.org

Review URL: https://codereview.chromium.org/28623002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229328 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229328