Add a guard in MojoBootstrap::OnClientLaunched()

Add a guard in MojoBootstrap::OnClientLaunched()

There is a race where the IO thread got error after the UI thread
launched a process. This CL rejects such a case.
This is OK as the error is eventually notified to the UI thread.

TEST=chromeos browser_tests with ChannelMojo being on.
R=viettrungluu@chromium.org
BUG=377980
Committed: https://crrev.com/0076bcf1a307e61d32dba4d94e2efa3f2643e3d4
Cr-Commit-Position: refs/heads/master@{#300589}

[Hajime Morrita, 2014-10-21 22:10:06]

Trung: PTAL?
I'm sweeping out remaining errors I'm seeing on
https://codereview.chromium.org/664413003/

[viettrungluu, 2014-10-21 22:17:17]

lgtm

[Hajime Morrita, 2014-10-21 22:39:50]

The CQ bit was checked by morrita@chromium.org

[commit-bot: I haz the power, 2014-10-21 22:41:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/671513011/1

[commit-bot: I haz the power, 2014-10-22 00:59:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-22 04:04:25]

Patchset 1 (id:??) landed as
https://crrev.com/0076bcf1a307e61d32dba4d94e2efa3f2643e3d4
Cr-Commit-Position: refs/heads/master@{#300589}