An accessibility tree update with two roots should be rejected.

An accessibility tree update with two roots should be rejected.

If an AXTreeUpdate had two root nodes, the second one would cause the
first one to be deleted, but that could leave dangling pointers in
|pending_nodes|. Fix this by first ensuring that |pending_nodes| is
always cleaned up when nodse are deleted, but also by specifically
returning an error if we encounter two root nodes since that should
never happen.

BUG=479743
Committed: https://crrev.com/e3b7faf115bcd4fec56b1658e60abe077d77f379
Cr-Commit-Position: refs/heads/master@{#332212}

[dmazzoni, 2015-05-27 17:02:31]

dmazzoni@chromium.org changed reviewers:
+ aboxhall@chromium.org

[aboxhall, 2015-06-01 16:24:56]

lgtm

[dmazzoni, 2015-06-01 16:46:50]

The CQ bit was checked by dmazzoni@chromium.org

[commit-bot: I haz the power, 2015-06-01 16:48:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1151393006/1

[commit-bot: I haz the power, 2015-06-01 17:56:45]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-01 17:57:32]

Patchset 1 (id:??) landed as
https://crrev.com/e3b7faf115bcd4fec56b1658e60abe077d77f379
Cr-Commit-Position: refs/heads/master@{#332212}