Remove is_main_frame and some of the navigation hacks for --site-per-process.

      These are no longer necessary now that we have RenderFrameHosts for all frames.

BUG=314791
TEST=Visit a cross-site iframe with --site-per-process.
R=nasko@chromium.org

Review URL: https://codereview.chromium.org/206953002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@258671 0039d316-1c4b-4281-b951-d872f2087c98