Fix a crash in ChildThread::ShutdownThread

From the bug report, it is the case when the child browser process
is not fully initialized (GPU thread is not initialized), but the
system decided to kill the process, ChildThread::ShutdownThread
doesn't check of g_child_thread is initialized or not, and it leads
to a SIGSEGV.

BUG=10412164
R=jam@chromium.org, klobag@chromium.org, michaelbai@chromium.org

Review URL: https://codereview.chromium.org/22914032

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225867 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225867