Cleanup ColorDistance

Remove all constructors, remove clampColor as we can just use makeRGBA.
Also make distance static so we don't need any state in this helper class.

BUG=

Review URL: https://codereview.chromium.org/112653005

git-svn-id: svn://svn.chromium.org/blink/trunk@164409 bbb929c8-8fbe-4397-9dbb-9b2b20218538