Add a move constructor and move assignment operator to Length

Add a move constructor and move assignment operator to Length for
performance.

This is inspired by the following WebKit revision by andersca@apple.com:
http://trac.webkit.org/changeset/156132

R=tkent@chromium.org

Review URL: https://codereview.chromium.org/296403003

git-svn-id: svn://svn.chromium.org/blink/trunk@174830 bbb929c8-8fbe-4397-9dbb-9b2b20218538