Cancel handler for JS unload handler prevents hang.

 Bug 6803309
 When the alert dialog raised by the JS unload handler was pressed
 outside the alert region, it was canceled and no response was
 sent back to WebViewCore. This captures the cancel event so that
 WebViewCore doesn't hang.

Change-Id: I0c125e759a252a803c2c77aa9533adef7fa82d3d