Remove WebTouchEvent.targetTouches

Remove this field as it is no longer used by Chromium. Also prepare the
way for removing |WebTouchEvent.changedTouches| by populating
|WebTouchEvent.touches| with *all* touch points, even those that are
released or cancelled, in WebTouchEventBuilder. The plugin consumer
downstream can resynthesize the changed touches on-demand.

This is part 2 of a 4-sided patch simplifying WebTouchEvent:
1) https://codereview.chromium.org/557863002/
2) This change
3) https://codereview.chromium.org/559693003/
4) https://codereview.chromium.org/560493003/

BUG=358132

Review URL: https://codereview.chromium.org/558773002

git-svn-id: svn://svn.chromium.org/blink/trunk@181775 bbb929c8-8fbe-4397-9dbb-9b2b20218538