[css-tables] Set table and cell widths dirty when section border changes

When borders collapse, section borders affect both the table width and
cell widths. Previously, tables and cells would use the old width when a
section's border changed.

BUG=613728

Review-Url: https://codereview.chromium.org/2013693002
Cr-Commit-Position: refs/heads/master@{#404994}
9 files changed