Fix thread DCHECK in child process ServiceManagerConnection

|child_binding_| was introduced by r461826 to be bound on the IO
thread. This CL ensures that it's also unbound on the same thread
during shutdown; otherwise there's a race and it might get
incorrectly unbound on the main thread, leading to a DCHECK in
developer builds.

BUG=716416
TBR=ben@chromium.org

Change-Id: I194d1446753d3fb976b21bb191e7a407e7f8c6e7
Reviewed-on: https://chromium-review.googlesource.com/491886Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#468405}

content/common/service_manager/service_manager_connection_impl.cc

@@ -207,6 +207,7 @@ class ServiceManagerConnectionImpl::IOThreadContext
 
     request_handlers_.clear();
     embedded_services_.clear();
+    child_binding_.Close();
   }
 
   void ClearConnectionFiltersOnIOThread() {