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

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252507 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252507