mojo: Fix crash in example WM when closing window.

mojo: Fix crash in example WM when closing window.

Avoid a crash caused by dereferencing an iterator after
erasing it.

BUG=415816
Committed: https://crrev.com/de4d067f40fa7d1b89cf77b0d3c642306569b47a
Cr-Commit-Position: refs/heads/master@{#295718}

[Daniel Erat, 2014-09-19 00:35:24]

derat@chromium.org changed reviewers:
+ aa@chromium.org

[Aaron Boodman, 2014-09-19 00:39:08]

Very gracious of you to not mention that I assigned you a bug that I myself
wrote. A few weeks ago.

LGTM

[Daniel Erat, 2014-09-19 00:39:49]

The CQ bit was checked by derat@chromium.org

[commit-bot: I haz the power, 2014-09-19 00:41:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/580353002/1

[commit-bot: I haz the power, 2014-09-19 06:42:59]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-09-19 06:43:01]

Exceeded time limit waiting for builds to trigger.

[Daniel Erat, 2014-09-19 15:39:09]

The CQ bit was checked by derat@chromium.org

[commit-bot: I haz the power, 2014-09-19 15:39:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/580353002/1

[commit-bot: I haz the power, 2014-09-19 16:15:33]

Committed patchset #1 (id:1) as ecef251ad56bb1121c8de5a8944a979dfd695a19

[commit-bot: I haz the power, 2014-09-19 16:16:14]

Patchset 1 (id:??) landed as
https://crrev.com/de4d067f40fa7d1b89cf77b0d3c642306569b47a
Cr-Commit-Position: refs/heads/master@{#295718}