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In GTK4, the GDestroyNotify that SelectFileDialogImplGtk used does not get fired
when the GtkFileChooser is closed (perhaps GTK is now doing some lazy
deletion?).  But the "destroy" signal does get fired, so move the event
re-enabling code there.

Also clean up some messy code.

BUG=876586
R=sky

Change-Id: I2d78ff292d167fe526bd5fb35aaf6296d32da1af
Reviewed-on: https://chromium-review.googlesource.com/1188585Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586078}