Allow PeerConnection to be garbage collected after close().

LayoutTests/fast/mediastream/RTCPeerConnection-stats-expected.txt

 Tests the RTCPeerConnection stats interface.
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
 PASS pc = new webkitRTCPeerConnection(null) did not throw exception.
 PASS pc.getStats(statsHandler1) did not throw exception.
 PASS statsHandler1 was called
 PASS status is non-null.
 PASS result.length is 0
 PASS getUserMedia({audio:true, video:true}, gotStream) did not throw exception.
 PASS Got a stream.
 PASS pc.getStats(statsHandler2) did not throw exception.
 PASS statsHandler2 was called
 PASS result.length is >= 2
 PASS timediff is >= 0
 PASS local.names().length is >= 1
 PASS local.names().indexOf("type") is >= 0
 PASS local.stat("type") is "audio"
 PASS timediff is >= 0
 PASS res.id is non-null.
 PASS res.type is non-null.
 PASS res.names().length is >= 1
 PASS res.names().indexOf("type") is >= 0
 PASS res.stat("type") is "audio"
 PASS status_g.namedItem(res.id) is non-null.
 PASS status_g[res.id] is non-null.
+PASS pc.close() did not throw exception.
+PASS pc.getStats(statsHandler3) did not throw exception.
+PASS statsHandler3 was called
+PASS result.length is >= 2
+PASS timediff is >= 0
+PASS local.names().length is >= 1
+PASS local.names().indexOf("type") is >= 0
+PASS local.stat("type") is "audio"
+PASS timediff is >= 0
+PASS res.id is non-null.
+PASS res.type is non-null.
+PASS res.names().length is >= 1
+PASS res.names().indexOf("type") is >= 0
+PASS res.stat("type") is "audio"
+PASS status_g.namedItem(res.id) is non-null.
+PASS status_g[res.id] is non-null.
 PASS successfullyParsed is true
 
 TEST COMPLETE