Clamp colspan to 8190

The RenderTableCell logic crashes if we try to set a
column index that would overflow the bitfield. The
assumption was that it would be rarely hit in the wild:
having such a big column index means a giant table, which
would be slow. Unfortunately, it seems like people (not to
mention fuzzers) are testing big values for colspan that
triggers the previous crash frequently. As a work-around,
this change clamps colspan - like rowspan - to 8190, which
should reduce greatly the amount of crashes.

BUG=386260

Review URL: https://codereview.chromium.org/393673002

git-svn-id: svn://svn.chromium.org/blink/trunk@178202 bbb929c8-8fbe-4397-9dbb-9b2b20218538