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

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

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/macros.h"
+#include "base/synchronization/lock.h"
+
+namespace mojo {
+namespace internal {
+
+// Similar to base::AutoLock, except that it does nothing if |lock| passed into
+// the constructor is null.
+class MayAutoLock {
+ public:
+  explicit MayAutoLock(base::Lock* lock) : lock_(lock) {
+    if (lock_)
+      lock_->Acquire();
+  }
+
+  ~MayAutoLock() {
+    if (lock_) {
+      lock_->AssertAcquired();
+      lock_->Release();
+    }
+  }
+
+ private:
+  base::Lock* lock_;
+  DISALLOW_COPY_AND_ASSIGN(MayAutoLock);
+};
+
+// Similar to base::AutoUnlock, except that it does nothing if |lock| passed
+// into the constructor is null.
+class MayAutoUnlock {
+ public:
+  explicit MayAutoUnlock(base::Lock* lock) : lock_(lock) {
+    if (lock_) {
+      lock_->AssertAcquired();
+      lock_->Release();
+    }
+  }
+
+  ~MayAutoUnlock() {
+    if (lock_)
+      lock_->Acquire();
+  }
+
+ private:
+  base::Lock* lock_;
+  DISALLOW_COPY_AND_ASSIGN(MayAutoUnlock);
+};
+
+}  // namespace internal
+}  // namespace mojo