Allow storing null navigations in CRWWKNavigationStates.

ios/web/web_state/ui/crw_wk_navigation_states_unittest.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/web/web_state/ui/crw_wk_navigation_states.h"
 
 #import <WebKit/WebKit.h>
 
 #import "base/mac/scoped_nsobject.h"
 #include "ios/web/web_state/navigation_context_impl.h"
@@ skipping 92 matching lines @@
   [states_ setContext:std::move(context2) forNavigation:navigation1_];
   EXPECT_FALSE([states_ contextForNavigation:navigation2_]);
   EXPECT_FALSE([states_ contextForNavigation:navigation3_]);
   ASSERT_TRUE([states_ contextForNavigation:navigation1_]);
   EXPECT_EQ(GURL(kTestUrl2),
             [states_ contextForNavigation:navigation1_]->GetUrl());
   EXPECT_FALSE([states_ contextForNavigation:navigation1_]->IsSameDocument());
   EXPECT_TRUE([states_ contextForNavigation:navigation1_]->IsErrorPage());
 }
 
+// Tests null WKNavigation object.
+TEST_F(CRWWKNavigationStatesTest, NullNavigation) {
+  // navigation_1 is the only navigation and it is the latest.
+  [states_ setState:WKNavigationState::REQUESTED forNavigation:navigation1_];
+  ASSERT_EQ(navigation1_, [states_ lastAddedNavigation]);
+  EXPECT_EQ(WKNavigationState::REQUESTED, [states_ lastAddedNavigationState]);
+
+  // null navigation is added later and hence the latest.
+  [states_ setState:WKNavigationState::STARTED forNavigation:nil];
+  EXPECT_FALSE([states_ lastAddedNavigation]);
+  EXPECT_EQ(WKNavigationState::STARTED, [states_ lastAddedNavigationState]);
+
+  // navigation_1 is the is the latest again after removing null navigation.
+  [states_ removeNavigation:nil];
+  ASSERT_EQ(navigation1_, [states_ lastAddedNavigation]);
+  EXPECT_EQ(WKNavigationState::REQUESTED, [states_ lastAddedNavigationState]);
+}
+
 }  // namespace web