Micro-optimise Length::initFromLength

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).

R=eseidel@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/14283003

git-svn-id: svn://svn.chromium.org/blink/trunk@148621 bbb929c8-8fbe-4397-9dbb-9b2b20218538
1 file changed