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
Review URL: https://chromiumcodereview.appspot.com/11155017
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@162217 0039d316-1c4b-4281-b951-d872f2087c98
f57c14b47c