message: Fix build break on Linux.
  Some scoped_lock were not migrated to absl::MutexLock. This CL fixes them.
diff: @@ -517,7 +517,7 @@ class SelectionMonitorImpl : public SelectionMonitorInterface, public Thread {
  SelectionInfo GetSelectionInfo() override {
  SelectionInfo info;
  {
  - scoped_lock l(&mutex_);
  + absl::MutexLock l(&mutex_);
  info = last_selection_info_;
  }
  return info;
  @@ -527,7 +527,7 @@ class SelectionMonitorImpl : public SelectionMonitorInterface, public Thread {
  void Run() override {
  while (!quit_) {
  if (!server_->checkConnection()) {
  - scoped_lock l(&mutex_);
  + absl::MutexLock l(&mutex_);
  last_selection_info_ = SelectionInfo();
  quit_ = true;
  break;
  @@ -538,7 +538,7 @@ class SelectionMonitorImpl : public SelectionMonitorInterface, public Thread {
  // X11 message is received. In order to interrupt, you can call
  // SendNoopEventMessage() method from other threads.
  if (server_->WaitForNextSelectionEvent(max_text_bytes_, &next_info)) {
  - scoped_lock l(&mutex_);
  + absl::MutexLock l(&mutex_);
  last_selection_info_ = next_info;
  }
  }
  