Fix pinchUpdate scale semantics

We expect the 'scale' value for GesturePinchUpdate to be a scale factor
(with 1.0 meaning no scaling).  The Cocoa API docs say you must add
1.0 to the magnification value to get the scale factor.

BUG=289887

Review URL: https://codereview.chromium.org/254783004

git-svn-id: svn://svn.chromium.org/blink/trunk@172763 bbb929c8-8fbe-4397-9dbb-9b2b20218538