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On Windows, fullscreen widgets kill themselves when losing focus,
which means that if they hang, the mere act of trying to display
the "hung page" dialog will kill the hung page.

Watch the widget to avoid a crash in this case.

This shouldn't be (and isn't) crashing on the Mac in this way, but
this makes the parallel changes on the Mac for symmetry.

BUG=811737
TEST=as in bug

Change-Id: I81f1797c3f5473b21eee2eed6c660e0f3acd1352
Reviewed-on: https://chromium-review.googlesource.com/961461Reviewed-by: Sidney San Martín <sdy@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543151}