Enable discard-input-to-moving-iframes intervention

Intent to implement and ship:

https://groups.google.com/a/chromium.org/d/msg/blink-dev/MzXscv7fT0w/ygq7qc6sBQAJ

The implementation landed behind a flag here:

https://chromium-review.googlesource.com/c/chromium/src/+/1686824

It has been in a few Canary versions without issue, and appears to
work as advertised in manual testing.

BUG=603193

Change-Id: I6e8737ef6de4444f887e986566c154f331869a5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1695965
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676608}