Add ability to return moved value from FunctorMessageHandler, Optional.

webrtc/base/optional.h

Index: webrtc/base/optional.h
diff --git a/webrtc/base/optional.h b/webrtc/base/optional.h
index 4d2b44e0cb62ed4d3f6cbf91c8f2649ba73c7a28..f5354ee0f25fdb6eabcf6fa367e3db64152681ee 100644
--- a/webrtc/base/optional.h
+++ b/webrtc/base/optional.h
@@ -239,6 +239,12 @@ class Optional final {
                       : default_val;
   }
 
+  // Dereference and move value.
+  T MoveValue() {
+    RTC_DCHECK(has_value_);
+    return std::move(value_);
+  }
+
   // Equality tests. Two Optionals are equal if they contain equivalent values,
   // or if they're both empty.
   friend bool operator==(const Optional& m1, const Optional& m2) {