Micro-optimize Length::initFromLength().
<http://webkit.org/b/115073>

From Blink r148621 by <timloh@chromium.org>:

This appears to improve html5-full-render by 1-2% on my system (gcc 4.6.3) by using memcpy
instead of copying members (and branching to copy the union).

* platform/Length.h:
(WebCore::Length::initFromLength):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149013 268f45cc-cd09-0410-ab3c-d52691b4dbfc