[Nav Experiment] Fix NavigationTestCase/testHistoryBackNavigation.

This EarlGrey test tests the behavior of renderer-initiated back
navigation. With WKBasedNavigationManager, renderer-initiated
back navigation happens completely in the WebProcess. Adding the wait
makes sure the Omnibox URL check is performed after the navigation
finishes.

Bug: 807428
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I636f4b991e6bcb901b09851de08b4fef6970002a
Reviewed-on: https://chromium-review.googlesource.com/894177
Commit-Queue: Danyao Wang <danyao@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533342}

ios/chrome/browser/web/navigation_egtest.mm

@@ -271,6 +271,7 @@ std::unique_ptr<net::test_server::HttpResponse> WindowLocationHashHandlers(
 
   // Tap the back button in the HTML and verify the first URL is loaded.
   [ChromeEarlGrey tapWebViewElementWithID:kGoBackID];
+  [ChromeEarlGrey waitForWebViewContainingText:"pony"];
   [[EarlGrey selectElementWithMatcher:OmniboxText(firstURL.GetContent())]
       assertWithMatcher:grey_notNil()];