Revert 252507 "Re-enable MetricsServiceTest.RegisterSyntheticTrial."

This test is failing on ~25% of win_rel try jobs and the CQ is suffering.

http://build.chromium.org/p/tryserver.chromium/builders/win_rel/builds/268519
http://build.chromium.org/p/tryserver.chromium/builders/win_rel/builds/268515
http://build.chromium.org/p/tryserver.chromium/builders/win_rel/builds/268511

> Re-enable MetricsServiceTest.RegisterSyntheticTrial.
> 
> The test passes locally and the failure stacktrace from
> the bug should have been fixed by http://crrev.com/240919.
> 
> BUG=320433
> 
> Review URL: https://codereview.chromium.org/174403003

TBR=asvitkine@chromium.org

Review URL: https://codereview.chromium.org/176423002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252821 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252821