Use correct proportion when computing the thumb length

This CL checks used total size before computing the thumb
size. If it's zero, we'll show no thumb.

BUG=421869

Review URL: https://codereview.chromium.org/662593004

git-svn-id: svn://svn.chromium.org/blink/trunk@183919 bbb929c8-8fbe-4397-9dbb-9b2b20218538