Revert 234881 "MetricsService API to support synthetic field trials"

[MetricsServiceTest.RegisterSyntheticTrial fails under ASAN.]

> MetricsService API to support synthetic field trials
> 
> Extend the MetricsService API to register synthetic field trials, where trial groups map to Chrome configuration states, e.g., values of a preference. These trials are included in UMA reports.
> 
> BUG=270275
> 
> Review URL: https://codereview.chromium.org/22340005

TBR=bengr@chromium.org

Review URL: https://codereview.chromium.org/61453003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234920 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234920