GitLab

Original change: https://codereview.chromium.org/2323103002/

Fix: On Android, the delegate callback called in DestroySubtree was
accessing AXTree::root() after we had deleted it but before we set it
to null. We need to clear it first and then destroy it.

BUG=646777
TBR=aboxhall,mmoroz

Review-Url: https://codereview.chromium.org/2340893002
Cr-Commit-Position: refs/heads/master@{#418698}