Disable offsetWidth optimization in perparation for disabling it on M30.

Unfortunately our offsetWidth optimization which made economist.com 2s faster
isn't quite correct.  It caused a bad regression on M30, which was noticed
in the Beta channel.

The code has differed some from the Beta branch, but in an effort to be
as safe as can be, I'm making a very-similar change on Trunk to the
change I'm about to roll into M30.

We will need to fix this optimization and re-enable it once it's safe
in order to get our 2s back on economist.com.

I expect this to slow down some benchmarks.

R=abarth@chromium.org, abarth, enne, tonyg
BUG=290399

Review URL: https://codereview.chromium.org/26355002

git-svn-id: svn://svn.chromium.org/blink/trunk@159046 bbb929c8-8fbe-4397-9dbb-9b2b20218538