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This CL makes it so that portal activation is blocked when the document
enters a beforeunload handler. If the unload proceeds, portal activation
will continue to be blocked

This must be implemented inside the renderer as the renderer can
initiate navigations which the browser won't hear about until after
beforeunload is confirmed. We do this by looking at the Document's
LoadEventProgress state.

Unfortunately, the current behavior is that, following a beforeunload,
this state would progress unconditionally to
kBeforeUnloadEventCompleted. This prevents us from distinguishing the
case where the navigation is canceled, which should once again allow
portal activation. This CL updates this state to only progress to the
(renamed for consistency and clarity) kBeforeUnloadEventCompleted state
only after we confirm the unload will proceed.

Bug: 1043764
Change-Id: I80363ccf2228e3dc5434486c95c62c17e00743aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2251104
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780470}