Mojo C++ bindings: change some std::unique_ptr<base::Lock> to base::Optional<base::Lock>.

mojo/public/cpp/bindings/connector.h

Index: mojo/public/cpp/bindings/connector.h
diff --git a/mojo/public/cpp/bindings/connector.h b/mojo/public/cpp/bindings/connector.h
index 1ccc79586b0eea0a152b282cd598b00e1f432c50..01e923679cd6d62ee83c34096c845504737f4168 100644
--- a/mojo/public/cpp/bindings/connector.h
+++ b/mojo/public/cpp/bindings/connector.h
@@ -11,6 +11,7 @@
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
+#include "base/optional.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread_checker.h"
 #include "mojo/public/cpp/bindings/bindings_export.h"
@@ -197,7 +198,7 @@ class MOJO_CPP_BINDINGS_EXPORT Connector
 
   // If sending messages is allowed from multiple threads, |lock_| is used to
   // protect modifications to |message_pipe_| and |drop_writes_|.
-  std::unique_ptr<base::Lock> lock_;
+  base::Optional<base::Lock> lock_;
 
   std::unique_ptr<SyncHandleWatcher> sync_watcher_;
   bool allow_woken_up_by_others_ = false;