Add StartSyncFlare to PasswordSyncableService.

BUG=95758

Review URL: https://codereview.chromium.org/183223003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255690 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255690