Check external_services_to_proxy for interfaces from external services

If a service appears in external_services_to_proxy, it is supposed to be
registered and stored in services_ as well. BindInterface() should not
query ServiceManager but instead should store this request through
ServiceNotFound for later query when that particular external service is
registered through RegisterExternalServices(..). Otherwise the binding
process to this external service (which only has earlier-registered
service_instance) will be silently dropped if the the client doesn't
set any disconnection handler.

Refer to go/ml-framework-standalone-binary-flakiness for more info.

Bug: b/168555534
Test: CQ

Change-Id: I2f791c924e1b23518704ad7237fc1e5624eaefd8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537975
Commit-Queue: Zhaoxin Liang <zxliang@google.com>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827706}

chromecast/external_mojo/public/cpp/external_mojo_broker.cc

@@ -5,6 +5,7 @@
 #include "chromecast/external_mojo/public/cpp/external_mojo_broker.h"
 
 #include <map>
+#include <set>
 #include <utility>
 
 #if defined(OS_LINUX) || defined(OS_CHROMEOS)
@@ -186,6 +187,9 @@ class ExternalMojoBroker::ConnectorImpl : public mojom::ExternalConnector {
       return;
     }
 
+    external_services_to_proxy_.insert(external_services_to_proxy.begin(),
+                                       external_services_to_proxy.end());
+
     for (const auto& service_name : external_services_to_proxy) {
       LOG(INFO) << "Register proxy for external " << service_name;
       mojo::PendingRemote<service_manager::mojom::Service> service_remote;
@@ -272,7 +276,9 @@ class ExternalMojoBroker::ConnectorImpl : public mojom::ExternalConnector {
       return;
     }
 
-    if (!connector_) {
+    auto service_proxy_it = external_services_to_proxy_.find(service_name);
+
+    if (!connector_ || service_proxy_it != external_services_to_proxy_.end()) {
       ServiceNotFound(service_name, interface_name, std::move(interface_pipe));
       return;
     }
@@ -349,6 +355,7 @@ class ExternalMojoBroker::ConnectorImpl : public mojom::ExternalConnector {
   std::unique_ptr<service_manager::Connector> connector_;
 
   mojo::ReceiverSet<mojom::ExternalConnector> receivers_;
+  std::set<std::string> external_services_to_proxy_;
   std::map<std::string, std::unique_ptr<ExternalServiceProxy>>
       registered_external_services_;