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This updates the CommitPending logic now that swap out happens at commit
time, ensuring that the old RFH is kept alive until the SwapOut ACK is
received.

It is now also possible to swap out without creating a proxy.

BUG=407160
TEST=Cross-site iframe is visible in --site-per-process mode.

Review URL: https://codereview.chromium.org/525583002

Cr-Commit-Position: refs/heads/master@{#297902}