Mark NavigationAndLoadCallbacksTest.FailedLoad flaky

Test NavigationAndLoadCallbacksTest.FailedLoad has been
flaky (probably since crrev.com/1065632), so mark it as
such until it is fixed.

Bug: 845879
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ibd43be2bc3b0c02feaf7c777bef1dd261a9705cd
Reviewed-on: https://chromium-review.googlesource.com/1069094
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561057}

ios/web/web_state/navigation_and_load_callbacks_inttest.mm

@@ -1262,7 +1262,8 @@ void TearDown() override {
 }
 
 // Tests failed load after the navigation is sucessfully finished.
-TEST_F(NavigationAndLoadCallbacksTest, FailedLoad) {
+// TODO(crbug.com/845879): test is flaky (probably since crrev.com/1065632).
+TEST_F(NavigationAndLoadCallbacksTest, FLAKY_FailedLoad) {
   GURL url = test_server_->GetURL("/exabyte_response");
 
   NavigationContext* context = nullptr;