Use LayoutUnit::fromFloatRound for setting scroll values

Change Elemenet::setScrollLeft and setScrollTop to use LayoutUnit::
fromFloatRound instead of lroundf. This ensures that no unnecessary type
conversion takers place and that the values are clamped properly.

R=leviw@chromium.org
BUG=393464
TEST=fast/scrolling/scroll-max-value.html

Review URL: https://codereview.chromium.org/400633005

git-svn-id: svn://svn.chromium.org/blink/trunk@178488 bbb929c8-8fbe-4397-9dbb-9b2b20218538