offsetWidth on a table cell with fixed width is wrong

TableCell's width is layout dependent, even when specified
via width: Npx in CSS.

Hopefully there are not other cases which are missing from this
optimization.  This code is very fragile as written.
We should make this a white-list instead of a black-list
for this optimization.

BUG=290399

Review URL: https://codereview.chromium.org/25086004

git-svn-id: svn://svn.chromium.org/blink/trunk@158467 bbb929c8-8fbe-4397-9dbb-9b2b20218538