Makes the process model heuristic for generated searches only apply in...

Makes the process model heuristic for generated searches only apply in --process-per-site, since it isn't relevant otherwise.  (This matters now that process-per-site-instance is the default.)

BUG=1256259
TEST=Visit a non-Google page, then run a search from the location bar.  The tab should switch renderer processes.

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@65 0039d316-1c4b-4281-b951-d872f2087c98