Use the pinch viewport offset for tap disambiguation (part 3).

Cleanup Blink after chromium part has landed.

This change is part 3 of 3
 (part 1 is Blink side change https://codereview.chromium.org/584893004)
 (part 2 is chromium side change https://codereview.chromium.org/595693002).

BUG=370470

Review URL: https://codereview.chromium.org/622263002

git-svn-id: svn://svn.chromium.org/blink/trunk@183207 bbb929c8-8fbe-4397-9dbb-9b2b20218538