Fix emulate-touch-event test to pass reliably

This test was failing because mousemove events are generated during touch emulation when preventDefault isn't called on the touch event.  I've filed BUG 168399 to track this (and some other) issues with how touch event emulation behaves, but fixed the test to properly test the current behavior by calling preventDefault on the touch event.

BUG=241665

Review URL: https://chromiumcodereview.appspot.com/23364007

git-svn-id: svn://svn.chromium.org/blink/trunk@157462 bbb929c8-8fbe-4397-9dbb-9b2b20218538