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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}