Move optional.h to webrtc/api/

We use Optional in our public API, so its header should be in
webrtc/api/.

BUG=webrtc:8205

Review-Url: https://codereview.webrtc.org/3011943002
Cr-Commit-Position: refs/heads/master@{#19693}
diff --git a/webrtc/api/optional.cc b/webrtc/api/optional.cc
new file mode 100644
index 0000000..ec86966
--- /dev/null
+++ b/webrtc/api/optional.cc
@@ -0,0 +1,25 @@
+/*
+ *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "webrtc/api/optional.h"
+
+namespace rtc {
+namespace optional_internal {
+
+#if RTC_HAS_ASAN
+
+void* FunctionThatDoesNothingImpl(void* x) {
+  return x;
+}
+
+#endif
+
+}  // namespace optional_internal
+}  // namespace rtc