Remove stale FIXME about HTMLPreElement.width type

The HTML spec says "attribute long width":
http://whatwg.org/html#HTMLPreElement-partial

In Gecko this IDL attribute is also a number, but in IE11 it's a string.

BUG=none
NOTRY=true

Review URL: https://codereview.chromium.org/264713002

git-svn-id: svn://svn.chromium.org/blink/trunk@173039 bbb929c8-8fbe-4397-9dbb-9b2b20218538