VariationsService: Support activating field trials from the server.

BUG=305779
TEST=New unit test.

Review URL: https://codereview.chromium.org/26129011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228009 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228009