Switch to use WaitForLoadStop to fix flakiness.

The use of TestNavigationManager was copied from old code.

Fixed: 1311145,1283050
Change-Id: I5832708828d3a3a1014a1a03d7af5ee84a952bd9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3556259
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#986375}

content/browser/back_forward_cache_browsertest.cc

@@ -683,13 +683,12 @@ void BackForwardCacheBrowserTest::NavigateAndBlock(GURL url,
   std::unique_ptr<URLLoaderInterceptor> url_interceptor =
       URLLoaderInterceptor::SetupRequestFailForURL(url,
                                                    net::ERR_BLOCKED_BY_CLIENT);
-  TestNavigationManager manager(web_contents(), url);
   if (history_offset) {
     shell()->GoBackOrForward(history_offset);
   } else {
     shell()->LoadURL(url);
   }
-  manager.WaitForNavigationFinished();
+  WaitForLoadStop(web_contents());
   ASSERT_EQ(current_frame_host()->GetLastCommittedURL(), url);
   ASSERT_TRUE(current_frame_host()->IsErrorDocument());
 }