Remove sync handling of IsSearchProviderInstalled on the browser UI thread.

This is to avoid a deadlock on Windows. It will be replaced by a handler
on the I/O thread in a subsequent patch.

BUG=38475
TEST=None

Review URL: http://codereview.chromium.org/3142029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@57063 0039d316-1c4b-4281-b951-d872f2087c98