Adding referrerPolicy externs.

Based on https://w3c.github.io/webappsec-referrer-policy/

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=227927697

externs/browser/w3c_dom2.js

@@ -1766,12 +1766,6 @@ HTMLImageElement.prototype.lowSrc;
  */
 HTMLImageElement.prototype.name;
 
-/**
- * @type {string}
- * @see https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/referrerPolicy
- */
-HTMLImageElement.prototype.referrerPolicy;
-
 /**
  * @type {string}
  * @implicitCast

externs/browser/w3c_referrer_policy.js

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2018 The Closure Compiler Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * @fileoverview Definitions for W3C's referrer policy specification.
+ * @see https://w3c.github.io/webappsec-referrer-policy/#referrer-policy-delivery
+ * @externs
+ */
+
+/** @type {string} */
+HTMLAnchorElement.prototype.referrerPolicy;
+
+/** @type {string} */
+HTMLAreaElement.prototype.referrerPolicy;
+
+/** @type {string} */
+HTMLImageElement.prototype.referrerPolicy;
+
+/** @type {string} */
+HTMLIFrameElement.prototype.referrerPolicy;
+
+/** @type {string} */
+HTMLLinkElement.prototype.referrerPolicy;