Add more appropriate assert_unreached to RTCPeerConnection-ice test

BUG=614963

Review-Url: https://codereview.chromium.org/2289183003
Cr-Commit-Position: refs/heads/master@{#415429}

third_party/WebKit/LayoutTests/fast/peerconnection/RTCPeerConnection-ice.html

@@ -53,7 +53,7 @@ function addIceCandidateSuccess4()
 
 function unexpectedCallback()
 {
-    calledCallbacks.push('unexpected');
+    assert_unreached('Unexpected callback');
 }
 
 function expectedTypeError(error)