Set the site instance in all cases when we start a new load. If you navigate away from the new tab page (or another DOMUI page) before the DOMUI page commits, the subsequent page will inherit the SiteInstance of the original page.

      BUG=12718,40575
TEST=included unit test
Review URL: http://codereview.chromium.org/1519025

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@43879 0039d316-1c4b-4281-b951-d872f2087c98