[iOS] Mark flaky ios_web_inttests as FLAKY_.

Affected tests:
- ErrorPageTest.BackForwardErrorPage
- WebStateObserverTest.FailedLoad

TBR: eugenebut@chromium.org
Bug: 954231, 954232
Change-Id: I1783732aa37b3ce7db3f9878a5342d1a5bc12271
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574018Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652219}

ios/web/web_state/error_page_inttest.mm

@@ -134,7 +134,7 @@ class ErrorPageTest
 
 // Tests that the error page is correctly displayed after navigating back to it
 // multiple times. See http://crbug.com/944037 .
-TEST_P(ErrorPageTest, BackForwardErrorPage) {
+TEST_P(ErrorPageTest, FLAKY_BackForwardErrorPage) {
   test::LoadUrl(web_state(), server_.GetURL("/close-socket"));
   ASSERT_TRUE(WaitForErrorText(web_state(), server_.GetURL("/close-socket")));
 

ios/web/web_state/web_state_observer_inttest.mm

@@ -2034,7 +2034,7 @@ TEST_P(WebStateObserverTest, DownloadNavigation) {
 }
 
 // Tests failed load after the navigation is sucessfully finished.
-TEST_P(WebStateObserverTest, FailedLoad) {
+TEST_P(WebStateObserverTest, FLAKY_FailedLoad) {
   GURL url = test_server_->GetURL("/exabyte_response");
 
   NavigationContext* context = nullptr;