[Presentation API] Resolve PresentationRequest::reconnect() with existing presentation connection i…

third_party/WebKit/LayoutTests/presentation/resources/presentation-service-mock.js

 /*
  * Mock implementation of mojo PresentationService.
  */
 
 "use strict";
 
 let presentationServiceMock = loadMojoModules(
     'presentationServiceMock',
     [
       'third_party/WebKit/public/platform/modules/presentation/presentation.mojom',
@@ skipping 11 matching lines @@
           this.bindingSet_ = new bindings.BindingSet(
               presentationService.PresentationService);
         }
 
         startSession(urls) {
           return Promise.resolve({
               sessionInfo: { url: urls[0], id: 'fakesession' },
               error: null,
           });
         }
+
+        joinSession(urls) {
+          return Promise.resolve({
+              sessionInfo: { url: urls[0], id: 'fakeSessionId' },
+              error: null,
+          });
+        }
       }
 
       return new PresentationServiceMock(mojo.frameInterfaces);
     });