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This was formerly used by layout tests for cross-frame communication by
setting the flag and polling its value. These layout tests have since
been rewritten to use a combination of load events and postMessage.

BUG=405846
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/495323003

Cr-Commit-Position: refs/heads/master@{#291589}