Remove WebTouchEvent.changedTouches

This field is no longer needed or in use by embedders, and can be
safely removed.

This is part 4 of a 4-sided patch simplifying WebTouchEvent:
1) https://codereview.chromium.org/557863002/
2) https://codereview.chromium.org/558773002/
3) https://codereview.chromium.org/559693003/
4) This change

BUG=358132

Review URL: https://codereview.chromium.org/560493003

git-svn-id: svn://svn.chromium.org/blink/trunk@181834 bbb929c8-8fbe-4397-9dbb-9b2b20218538