[css-tables] Clean up code that detects significant border changes

De-duplicate; narrow a condition; fix a method that did the opposite of
its name; add comments, etc.

No new tests because there _should_ be no behavior changes.

BUG=613728

Review-Url: https://codereview.chromium.org/2153283002
Cr-Commit-Position: refs/heads/master@{#406051}
6 files changed