Use the pinch viewport offset for tap disambiguation.

Account for the pinch vieport offset in the touch rectangle calculation
and pass the offset to the client. The client needs the touch rectangle
relative to both the main frame and to the screen.

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

BUG=370470

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=182534

Review URL: https://codereview.chromium.org/584893004

git-svn-id: svn://svn.chromium.org/blink/trunk@182646 bbb929c8-8fbe-4397-9dbb-9b2b20218538