Blink WebTouchPoint stores its location as WebFloatPoint, instead of WebPoint.

This is the second of three patches converting WebTouchPoint to use
WebFloatPoint instead of WebPoint.

The first patch (https://codereview.chromium.org/140973005) allows
chromium to accept WebFloatPoint (it just casts all the floats to
ints).

The third patch (https://codereview.chromium.org/148453012/) will
clean up the chromium side.

BUG=336807
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/149053002

git-svn-id: svn://svn.chromium.org/blink/trunk@168641 bbb929c8-8fbe-4397-9dbb-9b2b20218538