Suppress race in ChromeBrowserMainParts::SetupFieldTrials

TBR=jam@chromium.org
BUG=109863
TEST=waterfall.sh

Review URL: http://codereview.chromium.org/9188006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@117226 0039d316-1c4b-4281-b951-d872f2087c98