Mark SyncTest#testDisableAndEnableSync as flaky.

BUG=311091

Review URL: https://codereview.chromium.org/39933002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230738 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230738