Fix issue of localDescription and remoteDescription getter.

Source/modules/mediastream/RTCSessionDescription.idl

Index: Source/modules/mediastream/RTCSessionDescription.idl
diff --git a/Source/modules/mediastream/RTCSessionDescription.idl b/Source/modules/mediastream/RTCSessionDescription.idl
index 6c9f668933ed144f0c70d2ef7f6c1eba71281078..69840fbeff4631ea2de18691bbc0a6b55cec17c3 100644
--- a/Source/modules/mediastream/RTCSessionDescription.idl
+++ b/Source/modules/mediastream/RTCSessionDescription.idl
@@ -37,8 +37,10 @@ enum RTCSdpType {
 };
 
 [
+    ActiveDOMObject,
     GarbageCollected,
-    Constructor(optional RTCSessionDescriptionInit descriptionInitDict)
+    Constructor(optional RTCSessionDescriptionInit descriptionInitDict),
+    ConstructorCallWith=ExecutionContext,
 ] interface RTCSessionDescription {
     attribute RTCSdpType? type;
     attribute DOMString? sdp;