Add a move constructor and move assignment operator to KURL

Add a move constructor and move assignment operator to KURL for performance.
I loaded gmail.com and the new constructor / assignment operator were called
~1500 times.

R=tkent@chromium.org

Review URL: https://codereview.chromium.org/297223005

git-svn-id: svn://svn.chromium.org/blink/trunk@174829 bbb929c8-8fbe-4397-9dbb-9b2b20218538