OOPIF: Don't crash when running layout tests that have an OOP iframe.
This CLs fixes a few places in layout test code that were assuming
that the top-level frame is local, which was causing renderer crashes
when running a layout test containing a cross-site frame with
--site-per-process. For example, most tests under
http/tests/security/postMessage/ ran into this.
BUG=474792
TEST=http/tests/security/postMessage/* tests don't crash with --site-per-process
Review URL: https://codereview.chromium.org/
1073533002
Cr-Commit-Position: refs/heads/master@{#324311}