Clean up overloading of deltaX/deltaY in platform gesture events.

This is the last patch in the series to remove overloading
of deltaX/Y in gesture events.

BUG=143237

Review URL: https://codereview.chromium.org/23850013

git-svn-id: svn://svn.chromium.org/blink/trunk@159458 bbb929c8-8fbe-4397-9dbb-9b2b20218538