GitLab

- observe lifetime of the view and invalidate our weak pointer to it when it is being deleted
- clean up strange useage of AXUniqueId

TBR=slan@chromium.org

Bug: 859190
Change-Id: I15b45750ddecde7d20c4c91502411690e4025c07
Reviewed-on: https://chromium-review.googlesource.com/c/1316440
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605484}