cros: Fix crash in about:tracing

cros: Fix crash in about:tracing

We've seen some crashes in DebugDaemonClientImpl::OnRequestStopSystemTracing.
If debugd crashes or completes I/O back to Chrome too early we could NULL out
a scoped_ptr<> before the code expects it.  Null check the pointer.

BUG=155774
TEST=crash reports


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=162217

[James Cook, 2012-10-15 19:12:18]

Sam, PTAL.  Zel says there are some crashes in debugd that are being fixed, but
this NULL-check should fix a Chrome crash.

[James Cook, 2012-10-16 16:09:41]

Ping Sam?

[Sam Leffler, 2012-10-16 16:15:16]

lgtm

need to add a unit test for this case; can you please open an issue (and feel
free to assign to me)

[commit-bot: I haz the power, 2012-10-16 16:21:13]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jamescook@chromium.org/11155017/1

[commit-bot: I haz the power, 2012-10-16 16:21:25]

Presubmit check for 11155017-1 failed and returned exit status 1.


Running presubmit commit checks ...

** Presubmit ERRORS **
Missing LGTM from an OWNER for files in these directories:
    chromeos

Presubmit checks took 1.3s to calculate.

[James Cook, 2012-10-16 16:24:44]

Steven, can I get OWNERS approval for this null-check?

[stevenjb, 2012-10-16 16:45:07]

lgtm

[commit-bot: I haz the power, 2012-10-16 16:53:22]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jamescook@chromium.org/11155017/1

[commit-bot: I haz the power, 2012-10-16 19:46:21]

Change committed as 162217