Do not create a temporary string to append the SVGLength's unit

This is a cherry-pick of Benjamin Poulain's patch from WebKit r132807.

Review URL: https://codereview.chromium.org/369573003

git-svn-id: svn://svn.chromium.org/blink/trunk@177443 bbb929c8-8fbe-4397-9dbb-9b2b20218538