Calculate client positions from offsets.

Calculate client positions from offsets.
https://bugs.webkit.org/show_bug.cgi?id=73640

Reviewed by Tony Chang.

This change calculates client positions from offset positions at run time to
remove platform-dependent constants from this test.

* fast/events/offsetX-offsetY-expected.txt:
* fast/events/offsetX-offsetY.html:
* platform/chromium-win/fast/events/offsetX-offsetY-expected.txt: Removed.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121735 268f45cc-cd09-0410-ab3c-d52691b4dbfc