Implement throttling behavior for LocalStorage mojo messages.

mojo/public/cpp/bindings/interface_endpoint_client.h

Index: mojo/public/cpp/bindings/interface_endpoint_client.h
diff --git a/mojo/public/cpp/bindings/interface_endpoint_client.h b/mojo/public/cpp/bindings/interface_endpoint_client.h
index 6129d75bbf7511716d6a511bfaa5fa67a972b47d..88f7268eef0dd84ccb5233323fd85ef3963317e5 100644
--- a/mojo/public/cpp/bindings/interface_endpoint_client.h
+++ b/mojo/public/cpp/bindings/interface_endpoint_client.h
@@ -77,6 +77,11 @@ class MOJO_CPP_BINDINGS_EXPORT InterfaceEndpointClient
     return !async_responders_.empty() || !sync_responses_.empty();
   }
 
+  size_t pending_responder_count() const {
+    DCHECK(thread_checker_.CalledOnValidThread());
+    return async_responders_.size();
+  }
+
   AssociatedGroupController* group_controller() const {
     return handle_.group_controller();
   }