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https://github.com/mikewest/embedding-requires-opt-in suggests that we
shift the default behavior of <iframe>, et al. to require embedees to
opt-into embeddings. This would protect developers by default, and seems
like a reasonable state for us to aim towards.

This patch implements this behavior in AncestorThrottle behind a new
feature flag for experimentation.

Bug: 1153274
Change-Id: Ib164cfde00fef836b409e7a684734862141d030f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2563300Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: Antonio Sartori <antoniosartori@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832345}