Limit referer header's value to 4k.

If the `referer` header's value is a URL with a length of more than 4k, strip it down to an origin.

Fixes whatwg/fetch#903.

Author: mikewest

index.src.html

@@ -811,6 +811,10 @@ <h3 id="determine-requests-referrer" dfn export>
       Let <var>referrerURL</var> be the result of <a href="#strip-url">stripping
       <var>referrerSource</var> for use as a referrer.</a>
     </li>
+    <li>
+      If <var>referrerURL</var>'s length is greater than 4096, set <var>referrerURL</var>
+      to <var>referrerOrigin</var>.
+    </li>
     <li>
       Let <var>referrerOrigin</var> be the result of
       <a href="#strip-url">stripping <var>referrerSource</var> for use as a