Add a crash-hunting CHECK() in RenderProcessHostImpl::AddRoute()

This is an equivalent of DCHECK() in IDMap.
According to crbug.com/381990, this can occasionally crash.
I'd like to enable this to collect more data if it still crashes.
If you see this CHECK() being hit in the wild,
please revert this change and let me know.

BUG=381990, 415059
R=nasko@chromium.org, amineer@chromium.org

Review URL: https://codereview.chromium.org/647623003

Cr-Commit-Position: refs/heads/master@{#299146}

content/browser/renderer_host/render_process_host_impl.cc

@@ -920,6 +920,9 @@ ServiceRegistry* RenderProcessHostImpl::GetServiceRegistry() {
 void RenderProcessHostImpl::AddRoute(
     int32 routing_id,
     IPC::Listener* listener) {
+  // We already have DCHECK() in IDMap. This is for chasing
+  // crbug.com/415059 and can be removed after fixing it.
+  CHECK(!listeners_.Lookup(routing_id));
   listeners_.AddWithID(listener, routing_id);
 }