Fix scrollWidth/Height to not truncate scroll offset

Fix bug in Element::scrollWidth/scrollHeight where the scroll offset
was truncated to an integer value before being adjusted for zoom.

TEST=fast/sub-pixel/clientwidth-matches-scrollwidth-for-float.html
BUG=381039

Review URL: https://codereview.chromium.org/319633004

git-svn-id: svn://svn.chromium.org/blink/trunk@175698 bbb929c8-8fbe-4397-9dbb-9b2b20218538