Make mock WebChannel less invasive.

The WebChannel mocking now calls the original functions for
addEventListener, removeEventListener, and dispatchEvent. This means that
it won't interfere with other uses of these APIs.

One example of such a use is the popstate event: UrlManager.test.js has
a test which makes sure that the URL state is restored when going
backwards in history. Using a mocked WebChannel in that test broke it
because the popstate event was no longer being dispatched.

src/test/fixtures/mocks/web-channel.js

@@ -16,25 +16,30 @@ import type {
 export function mockWebChannel() {
   const messagesSentToBrowser = [];
   const listeners = [];
+  const originalAddEventListener = window.addEventListener;
+  const originalRemoveEventListener = window.removeEventListener;
+  const originalDispatchEvent = window.dispatchEvent;
   let onMessageToChrome = null;
 
   jest
     .spyOn(window, 'addEventListener')
-    .mockImplementation((name, listener) => {
+    .mockImplementation((name, listener, options) => {
       if (name === 'WebChannelMessageToContent') {
         listeners.push(listener);
       }
+      originalAddEventListener.call(window, name, listener, options);
     });
 
   jest
     .spyOn(window, 'removeEventListener')
-    .mockImplementation((name, listener) => {
+    .mockImplementation((name, listener, options) => {
       if (name === 'WebChannelMessageToContent') {
         const index = listeners.indexOf(listener);
         if (index !== -1) {
           listeners.splice(index, 1);
         }
       }
+      originalRemoveEventListener.call(window, name, listener, options);
     });
 
   jest.spyOn(window, 'dispatchEvent').mockImplementation((event) => {
@@ -46,6 +51,8 @@ export function mockWebChannel() {
       if (onMessageToChrome) {
         onMessageToChrome(JSON.parse(event.detail).message);
       }
+    } else {
+      originalDispatchEvent.call(window, event);
     }
   });