Do not transform scroll event ordinal values

We don't need to transform scroll event ordinal values anymore as
we now interpret them as DIPs across the whole stack. Otherwise,
high-DPI devices (e.g. Pixel) would see the overscroll sensitivity
downscaled by the device scale factor.

Contributed by sheckylin@chromium.org

BUG=300059
TEST=Make sure it takes roughly the same distance to scroll on the
touchpad to trigger the back/forward history navigation gesture on
both Pixel and other low-res devices.

Review URL: https://codereview.chromium.org/25373005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226767 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226767