Safeguard beforeunload cancel by preventDefault behind flag

We want to put the new behavior (can cancel unload by calling
preventDefault on beforeunload event) behind a feature flag. There is a
low change of causing regressions, but we want to make this behavior
easy to disable when needed.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/4133847/comment/14522696_e33a1dfe/

Fixed: 866818
Change-Id: I8dac44930b5d1afef6e56415cfbb99aa74e43a07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4236391
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Di Zhang <dizhangg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1104727}