Fix layout test failure with fast/events/touch/touch-stale-node-crash.html

This layout test is currently failing due to timing out in DRT.

The issue is that the test sends a down, up, down sequence quickly. For
each down event, we post a PREVENT_DEFAULT_TIMEOUT message to the
WebView's message handler. WebCore responds to the first touch event and
we update the mPreventDefault state variable correctly. The second touch down
resets mPreventDefault as it's the start of a new touch sequence and a second touch down is posted
to the WebCore thread.

At this point we still have the first TIMEOUT message in the WebView queue. The problem occurs
when the WebView processes this timeout message before the WebCore thread processes the second
touch down message. In this case the WebView clears the WebCore thread's message queue and instead posts
touch cancel events, erroneously removing the second touch event. This timeout message should not have been
processed as it was associated with the first touch down that had already been completed. Without the
second touch the test never completes.

The fix is to remove PREVENT_DEFAULT_TIMEOUT messages from the queue before starting
a new touch sequence.

Change-Id: Ief054239529d710a79a0e58a589bd7a92434dbf2