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This flag takes precedence over --site-per-process and other
site isolation modes.

BUG=688617
TEST=Cross-site subframes work with --single-process --site-per-process
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2964033002
Cr-Commit-Position: refs/heads/master@{#485292}