Make fixed-width elements not center by default.

e223e0c6d51f027196a2597b556641328a65d4b5 accidentally
sent all elements down the margin:auto codepath.
The patch incorrectly removed just the
"&& containingBlockStyle->textAlign() == WEBKIT_CENTER"
instead of the whole clause.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/760143002