Gracefully handle tab update events for closing tabs.

Gracefully handle tab update events for closing tabs.

When we close a tab that shows a modal dialog, and another tab has a modal
dialog pending, we'll end up deactivating the first tab after closing it. In
that case, just ignore the event.

BUG=484550
R=kalman@chromium.org

Committed: https://crrev.com/d13e9be5142a6df82fa9fc12ec4708526ce83f38
Cr-Commit-Position: refs/heads/master@{#329541}

[not at google - send to devlin, 2015-05-12 20:46:27]

lgtm

[jochen (gone - plz use gerrit), 2015-05-13 00:24:05]

The CQ bit was checked by jochen@chromium.org

[commit-bot: I haz the power, 2015-05-13 00:25:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1141493002/1

[commit-bot: I haz the power, 2015-05-13 00:29:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-13 00:30:52]

Patchset 1 (id:??) landed as
https://crrev.com/d13e9be5142a6df82fa9fc12ec4708526ce83f38
Cr-Commit-Position: refs/heads/master@{#329541}