Use the URL passed to navigator.share().

third_party/WebKit/LayoutTests/webshare/resources/mock-share-service.js

 'use strict';
 
 let mockShareService = loadMojoModules(
     'mockShareService',
     ['mojo/public/js/router',
      'third_party/WebKit/public/platform/modules/webshare/webshare.mojom',
     ]).then(mojo => {
   let [router, webshare] = mojo.modules;
 
   class MockShareService extends webshare.ShareService.stubClass {
@@ skipping 10 matching lines @@
       this.shareResultQueue_ = [];
 
       return new Promise((resolve, reject) => {this.reject_ = reject});
     }
 
     connect_(handle) {
       this.router_ = new router.Router(handle);
       this.router_.setIncomingReceiver(this);
     }
 
-    share(title, text) {
+    share(title, text, url) {
       let callback = null;
       let result = new Promise(resolve => {callback = resolve;});
 
       if (!this.shareResultQueue_.length) {
         this.reject_('Unexpected call to mojo share method');
         return result;
       }
 
-      let expectedTitle, expectedText, error;
-      [expectedTitle, expectedText, error] = this.shareResultQueue_.shift();
+      let [expectedTitle, expectedText, expectedUrl, error] =
+          this.shareResultQueue_.shift();
       try {
         assert_equals(title, expectedTitle);
         assert_equals(text, expectedText);
+        assert_equals(url.url, expectedUrl);
       } catch (e) {
         this.reject_(e);
         return result;
       }
       callback({error: error});
 
       return result;
     }
 
-    pushShareResult(expectedTitle, expectedText, result) {
-      this.shareResultQueue_.push([expectedTitle, expectedText, result]);
+    pushShareResult(expectedTitle, expectedText, expectedUrl, result) {
+      this.shareResultQueue_.push(
+          [expectedTitle, expectedText, expectedUrl, result]);
     }
   }
   return new MockShareService(mojo.frameInterfaces);
 });
 
 function share_test(func, name, properties) {
   promise_test(t => mockShareService.then(mock => {
     let mockPromise = mock.init_();
     return Promise.race([func(t, mock), mockPromise]);
   }), name, properties);
 }
 
 // Copied from resources/bluetooth/bluetooth-helpers.js.
 function callWithKeyDown(functionCalledOnKeyPress) {
   return new Promise(resolve => {
     function onKeyPress() {
       document.removeEventListener('keypress', onKeyPress, false);
       resolve(functionCalledOnKeyPress());
     }
     document.addEventListener('keypress', onKeyPress, false);
 
     eventSender.keyDown(' ', []);
   });
 }