Change scrollWidth/Height/Left/Top to LayoutUnit for highdpi

Follow up to the change that exposed double precision values for the
[offset|client|scroll][Width|Height|Top|Left] values by switching the
types for the internal methods to LayoutUnits thereby improving high DPI
and zoomed content scroll precision.

BUG=360889
R=dglazkov@chromium.org, cpu@chromium.org

Review URL: https://codereview.chromium.org/291193010

git-svn-id: svn://svn.chromium.org/blink/trunk@174729 bbb929c8-8fbe-4397-9dbb-9b2b20218538