arc-bridge: Make ArcAppModelBuilderTest more reliable

components/arc/test/fake_app_instance.h

Index: components/arc/test/fake_app_instance.h
diff --git a/components/arc/test/fake_app_instance.h b/components/arc/test/fake_app_instance.h
index 97db901092fde5944baeaf6d55aa0c5f957d110d..0f48bf9316601ff8a2fc45a12ba7ce83df84f95c 100644
--- a/components/arc/test/fake_app_instance.h
+++ b/components/arc/test/fake_app_instance.h
@@ -85,6 +85,10 @@ class FakeAppInstance : public AppInstance {
     return icon_requests_;
   }
 
+  // This method can be called on tests when a method is intended to
+  // be called across a Mojo proxy.
+  void WaitForIncomingMethodCall();
+
  private:
   // Mojo endpoints.
   mojo::Binding<AppInstance> binding_;