Introduce url::Origin to represent Web Origin.

Introduce url::Origin to represent a serialized Web Origin defined in RFC6455.
This class wraps a string representation of blink-side SecurityOrigin object.

BUG=339373
R=tyoshino@chromium.org, ricea@chromium.org

Review URL: https://codereview.chromium.org/170843007

git-svn-id: https://src.chromium.org/chrome/trunk/src/url@256789 4ff67af0-8c30-449e-8e8b-ad334ec8d88c