Fix issue of localDescription and remoteDescription getter.

LayoutTests/fast/mediastream/RTCPeerConnection-localDescription.html

Index: LayoutTests/fast/mediastream/RTCPeerConnection-localDescription.html
diff --git a/LayoutTests/fast/mediastream/RTCPeerConnection-localDescription.html b/LayoutTests/fast/mediastream/RTCPeerConnection-localDescription.html
index f5775886c4618a29cec1b657a2f089c15fe9efd1..9aabdf1981bd1f84c135cbc8377cce09866e7e48 100644
--- a/LayoutTests/fast/mediastream/RTCPeerConnection-localDescription.html
+++ b/LayoutTests/fast/mediastream/RTCPeerConnection-localDescription.html
@@ -8,6 +8,7 @@
 description("Tests RTCPeerConnection localDescription.");
 
 var pc = null;
+var observation = null;
 
 function requestFailed2()
 {
@@ -38,8 +39,13 @@ function requestSucceeded1()
 {
     testPassed('requestSucceeded was called.');
 
-    sessionDescription = new RTCSessionDescription({type:"answer", sdp:"remote"});
-    shouldNotThrow('pc.setLocalDescription(sessionDescription, requestSucceeded2, requestFailed2);');
+    shouldBeTrue('pc.localDescription === sessionDescription');
+    observation = internals.observeGC(pc.localDescription);
+    asyncGC(function() {
+        shouldBeFalse('observation.wasCollected');
+        sessionDescription = new RTCSessionDescription({type:"answer", sdp:"remote"});
+        shouldNotThrow('pc.setLocalDescription(sessionDescription, requestSucceeded2, requestFailed2);');
+    });
 }
 
 pc = new webkitRTCPeerConnection(null, null);