Fixed bug preventing disabling the default search provider.

BUG=67180
TEST=Set the "DefaultSearchProviderEnabled" policy to false. Type keywords on the omnibox. No searches should be performed.


Review URL: http://codereview.chromium.org/7150012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@89219 0039d316-1c4b-4281-b951-d872f2087c98