Use the URL passed to navigator.share().

third_party/WebKit/public/platform/modules/webshare/webshare.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [JavaPackage="org.chromium.mojom.webshare"]
 module blink.mojom;
 
+import "url/mojo/url.mojom";
+
 interface ShareService {
   // The web contents has requested to send a share text action. Returns null on
   // success, or an error message on failure.
   // TODO(mgiuca): Return an error enum, instead of a string (so the actual
   // error messages are generated in a single place, rather than many service
   // handlers).
-  Share(string title, string text) => (string? error);
+  Share(string title, string text, url.mojom.Url url) => (string? error);
 };