Fix flaky NewTabPageTestCase.testOmnibox

[google_landing_controller scrolledToTop] is set at the begining of
animation.
This make taps done just after another tap flaky.
Add a wait on animateHeader to wait until animation is finished.

BUG=None

Review-Url: https://codereview.chromium.org/2606023003
Cr-Commit-Position: refs/heads/master@{#440966}

ios/chrome/browser/ui/ntp/google_landing_controller.mm

@@ -1674,10 +1674,16 @@ void SearchEngineObserver::OnTemplateURLServiceChanged() {
   _most_visited_sites->AddOrRemoveBlacklistedUrl(url, false);
 }
 
+#pragma mark - GoogleLandingController (ExposedForTesting) methods.
+
 - (BOOL)scrolledToTop {
   return _scrolledToTop;
 }
 
+- (BOOL)animateHeader {
+  return _animateHeader;
+}
+
 #pragma mark - OverscrollActionsControllerDelegate
 
 - (void)overscrollActionsController:(OverscrollActionsController*)controller

ios/chrome/browser/ui/ntp/new_tab_page_egtest.mm

@@ -28,6 +28,7 @@
 
 @interface GoogleLandingController (ExposedForTesting)
 - (BOOL)scrolledToTop;
+- (BOOL)animateHeader;
 @end
 
 namespace {
@@ -86,6 +87,11 @@ void AssertNTPScrolledToTop(bool scrolledToTop) {
       chrome_test_util::GetCurrentNewTabPageController();
   GoogleLandingController* google_landing_controller =
       [ntp_controller googleLandingController];
+  [[GREYCondition
+      conditionWithName:@"Wait for end of animation."
+                  block:^BOOL {
+                    return ![google_landing_controller animateHeader];
+                  }] waitWithTimeout:testing::kWaitForUIElementTimeout];
   GREYAssertTrue([google_landing_controller scrolledToTop] == scrolledToTop,
                  @"scrolledToTop_ does not match expected value");
 }