Enable the CrossSiteIframe test for --site-per-process to catch regressions.

Also removes an incorrect DCHECK from NavigatorImpl.

BUG=99379
TEST=SitePerProcessBrowserTest.CrossSiteIframe passes.
R=nasko@chromium.org

Review URL: https://codereview.chromium.org/147363007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250787 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250787