[ MimeHandlerView ] Reset beforeunload binding

When a new BeforeUnloadControl binding is being bound we should clear the old
binding. It is possible to try to bind an existing binding when the page is
refreshed.

TBR=wjmaclean@chromium.org

Bug: 659750
Change-Id: I1b41deb91e32cf394e9fa9c53de75c69141a8da5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1607503
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658893}
1 file changed