Add unit tests for ArcBridgeService::RemoveObserver()

components/arc/arc_bridge_service.h

Index: components/arc/arc_bridge_service.h
diff --git a/components/arc/arc_bridge_service.h b/components/arc/arc_bridge_service.h
index fb2ef4299b3e59025447f9d5e8c67bca2b6ba100..cd6793fc80dfb19089f001332dee5ec8cfdf1f13 100644
--- a/components/arc/arc_bridge_service.h
+++ b/components/arc/arc_bridge_service.h
@@ -177,7 +177,8 @@ class ArcBridgeService : public mojom::ArcBridgeHost {
   virtual void Shutdown() = 0;
 
   // Adds or removes observers. This can only be called on the thread that this
-  // class was created on.
+  // class was created on. RemoveObserver does nothing if |observer| is not in
+  // the list.
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);