Made testCachingBehaviorOnOpenNewTab more robust.

ios/chrome/browser/web/cache_egtest.mm

Index: ios/chrome/browser/web/cache_egtest.mm
diff --git a/ios/chrome/browser/web/cache_egtest.mm b/ios/chrome/browser/web/cache_egtest.mm
index c097c40b3947cdc7c99363b816d79ebdf28e2333..9b5f9728eb7c9573351db7d232bbd9ecc9df327e 100644
--- a/ios/chrome/browser/web/cache_egtest.mm
+++ b/ios/chrome/browser/web/cache_egtest.mm
@@ -16,6 +16,7 @@
 #include "ios/chrome/test/app/history_test_util.h"
 #include "ios/chrome/test/app/navigation_test_util.h"
 #include "ios/chrome/test/app/web_view_interaction_test_util.h"
+#import "ios/chrome/test/earl_grey/chrome_assertions.h"
 #import "ios/chrome/test/earl_grey/chrome_earl_grey.h"
 #import "ios/chrome/test/earl_grey/chrome_matchers.h"
 #import "ios/chrome/test/earl_grey/chrome_test_case.h"
@@ -243,6 +244,7 @@ class ScopedBlockPopupsPref {
   // first allow popups.
   ScopedBlockPopupsPref prefSetter(CONTENT_SETTING_ALLOW);
   chrome_test_util::TapWebViewElementWithId(kCacheTestLinkID);
+  chrome_test_util::AssertMainTabCount(2);
   [ChromeEarlGrey waitForPageToFinishLoading];
   [[EarlGrey selectElementWithMatcher:WebViewContainingText("First Page")]
       assertWithMatcher:grey_notNil()];