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If the edit distance is big enough, the ratio could be less than 0.
This CL fixes the bug.

Bug: 990684
Change-Id: I61e536aff49cef3cd5e844b816f8d3e6662f0e88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2068228Reviewed-by: Rachel Wong <wrong@chromium.org>
Commit-Queue: Thanh Nguyen <thanhdng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743878}