Mojo C++ bindings: remove the lock in MultiplexRouter if it only serves a single interface.

mojo/public/cpp/bindings/lib/multiplex_router.h

Index: mojo/public/cpp/bindings/lib/multiplex_router.h
diff --git a/mojo/public/cpp/bindings/lib/multiplex_router.h b/mojo/public/cpp/bindings/lib/multiplex_router.h
index 84272d3aef4dd66193b54feb0f17d2428e71a679..26de3df13b9a12d55c9fd9991c67c301e873c85b 100644
--- a/mojo/public/cpp/bindings/lib/multiplex_router.h
+++ b/mojo/public/cpp/bindings/lib/multiplex_router.h
@@ -56,10 +56,22 @@ class MultiplexRouter
       public AssociatedGroupController,
       public PipeControlMessageHandlerDelegate {
  public:
+  enum Config {
+    // There is only the master interface running on this router. Please note
+    // that because of interface versioning, the other side of the message pipe
+    // may use a newer master interface definition which passes associated
+    // interfaces. In that case, this router may still receive pipe control
+    // messages or messages targetting associated interfaces.
+    SINGLE_INTERFACE,
+    // There may be associated interfaces running on this router.
+    MULTI_INTERFACE
+  };
+
   // If |set_interface_id_namespace_bit| is true, the interface IDs generated by
   // this router will have the highest bit set.
-  MultiplexRouter(bool set_interface_id_namespace_bit,
-                  ScopedMessagePipeHandle message_pipe,
+  MultiplexRouter(ScopedMessagePipeHandle message_pipe,
+                  Config config,
+                  bool set_interface_id_namespace_bit,
                   scoped_refptr<base::SingleThreadTaskRunner> runner);
 
   // Sets the master interface name for this router. Only used when reporting
@@ -205,6 +217,8 @@ class MultiplexRouter
 
   InterfaceEndpoint* FindOrInsertEndpoint(InterfaceId id, bool* inserted);
 
+  void AssertLockAcquired();
+
   // Whether to set the namespace bit when generating interface IDs. Please see
   // comments of kInterfaceIdNamespaceMask.
   const bool set_interface_id_namespace_bit_;
@@ -220,7 +234,8 @@ class MultiplexRouter
   base::ThreadChecker thread_checker_;
 
   // Protects the following members.
-  mutable base::Lock lock_;
+  // Sets to nullptr in Config::SINGLE_INTERFACE mode.
+  std::unique_ptr<base::Lock> lock_;
   PipeControlMessageHandler control_message_handler_;
   PipeControlMessageProxy control_message_proxy_;