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This change replaces base::Unretained with a weak ptr everywhere where
there's possible a doubt that something could go wrong.

In particular, it uses weak ptrs for the try_done and all_done callbacks
of batch element checkers, since they can be called after the checker
has been deleted, even if it's unlikely.

This change also moves the script tracker in the Controller to make it
the last owned type to be deleted, as the script tracker will access
everything that exists in the controller, through actions. It's best to
delete it first.

Finally, this change adds null checks for ui_delegate_ in
android_ui_controller.cc, as the controller can delete itself separate
from any UI decisions. In this case, it sets ui_delegate_ to null.

Bug: 806868
Change-Id: Ie2e11ea54a1bdf80c4bc80d79a158a1658bc0d1b
Reviewed-on: https://chromium-review.googlesource.com/c/1353942
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612264}