Use correct proportion when computing the thumb length
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@183919 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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