Support process swaps for cross-site iframes with --site-per-process.

Until further support lands, any cross-site iframes will be displayed
as top-level pages when the --site-per-process flag is passed.

BUG=314791
TEST=Cross-site iframe causes a process swap when flag is passed.
R=jochen@chromium.org, nasko@chromium.org

Review URL: https://codereview.chromium.org/108663007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241600 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241600