Fix nullptr dereference in MojoShellConnectionImpl

Fix nullptr dereference in MojoShellConnectionImpl

ShutdownOnIOThread may be called more than once. If this happens,
the MessageLoop observer will be null during the second call.
This ensures that we don't dereference it in that case.

BUG=None
TBR=ben@chromium.org

Committed: https://crrev.com/6854da52a2abf8b9f1b56863eb9f6e30ea8a52e5
Cr-Commit-Position: refs/heads/master@{#412768}

[Ken Rockot(use gerrit already), 2016-08-18 06:31:27]

The CQ bit was checked by rockot@chromium.org

[commit-bot: I haz the power, 2016-08-18 06:31:41]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-08-18 08:16:18]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-08-18 08:18:51]

Description was changed from

==========
Fix nullptr dereference in MojoShellConnectionImpl

ShutdownOnIOThread may be called more than once. If this happens,
the MessageLoop observer will be null during the second call.
This ensures that we don't dereference it in that case.

BUG=None
TBR=ben@chromium.org
==========

to

==========
Fix nullptr dereference in MojoShellConnectionImpl

ShutdownOnIOThread may be called more than once. If this happens,
the MessageLoop observer will be null during the second call.
This ensures that we don't dereference it in that case.

BUG=None
TBR=ben@chromium.org

Committed: https://crrev.com/6854da52a2abf8b9f1b56863eb9f6e30ea8a52e5
Cr-Commit-Position: refs/heads/master@{#412768}
==========

[commit-bot: I haz the power, 2016-08-18 08:18:52]

Patchset 1 (id:??) landed as
https://crrev.com/6854da52a2abf8b9f1b56863eb9f6e30ea8a52e5
Cr-Commit-Position: refs/heads/master@{#412768}