Add DVLOG with child process id in mojo shell.

services/shell/runner/host/child_process_host.cc

Index: services/shell/runner/host/child_process_host.cc
diff --git a/services/shell/runner/host/child_process_host.cc b/services/shell/runner/host/child_process_host.cc
index 30195b5404f6da9fc8c06e5386c1f7ecddb4a20b..cc28e102ea1b29054eb1ce1e8f2187240bfb8447 100644
--- a/services/shell/runner/host/child_process_host.cc
+++ b/services/shell/runner/host/child_process_host.cc
@@ -199,6 +199,8 @@ void ChildProcessHost::DoLaunch(
   }
 
   if (child_process_.IsValid()) {
+    DVLOG(0) << "Launched child process '" << target_.instance()

[Ken Rockot(use gerrit already), 2016/07/29 17:48:00]

I think this is a reasonable thing to do. You may want to log more than just the
instance though. Might as well log name, user_id, and instance?

[kylechar, 2016/07/29 17:59:34]

Yep, they could definitely come in handy. I added the extra fields.

+             << "' pid=" << child_process_.Pid();
     if (mojo_ipc_channel_.get()) {
       mojo_ipc_channel_->ChildProcessLaunched();
       mojo::edk::ChildProcessLaunched(