MouseEvent.offsetX/Y should just return 0,0 for simulated clicks.

There's no need to compute the exact target-relative coordinates for
simulated mouse events, e.g those fired by HTMLElement.click().

The offsetX/Y properties are not supported by Firefox.

This CL merges http://trac.webkit.org/changeset/164871/ by kling, which made
WebKit go fast on Speedometer.

BUG=380173

Review URL: https://codereview.chromium.org/316733004

git-svn-id: svn://svn.chromium.org/blink/trunk@175426 bbb929c8-8fbe-4397-9dbb-9b2b20218538