DOMUI prefs: Fix a race condition in pushing search engine information to DOMUI

BUG=None
TEST=Load DOMUI options, go to another page, load DOMUI options again, and manage search engines; the table should not be empty.

Review URL: http://codereview.chromium.org/3116023

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@57077 0039d316-1c4b-4281-b951-d872f2087c98