Added the document referrer to the public WebDocument interface

There were existing methods to determine what the referrer should be for
a request sent in the context of the document but no way to get the
referrer of the current document from outside of Blink.

We need this to be able to do reporting of UMA metrics for pages that
came from a websearch click.

BUG=309021

Review URL: https://codereview.chromium.org/32423002

git-svn-id: svn://svn.chromium.org/blink/trunk@160237 bbb929c8-8fbe-4397-9dbb-9b2b20218538