Allow storing null navigations in CRWWKNavigationStates.

ios/web/web_state/ui/crw_wk_navigation_states.h

 // 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.
 
 #ifndef IOS_WEB_WEB_STATE_UI_CRW_WK_NAVIGATION_STATES_H_
 #define IOS_WEB_WEB_STATE_UI_CRW_WK_NAVIGATION_STATES_H_
 
 #include <memory>
 
 #import <Foundation/Foundation.h>
@@ skipping 23 matching lines @@
   COMMITTED,
   // WKNavigation passed to |webView:didFinishNavigation:|.
   FINISHED,
   // WKNavigation passed to |webView:didFailNavigation:withError:|.
   FAILED,
 };
 
 }  // namespace web
 
 // Stores states and navigation contexts for WKNavigation objects.
-// Allows looking up for last added navigation object.
+// Allows looking up for last added navigation object. null WKNavigation is a
+// valid navigation and treated as a unique key. WKWebView passes null
+// WKNavigation to WKNavigationDelegate callbacks if navigation represents
+// window opening action.
 @interface CRWWKNavigationStates : NSObject
 
 // Adds a new navigation if it was not added yet. If navigation was already
 // added then updates state for existing navigation. Updating state does not
 // affect the result of |lastAddedNavigation| method. New added navigations
 // should have WKNavigationState::REQUESTED, WKNavigationState::STARTED or
 // WKNavigationState::COMMITTED state. |navigation| will be held as a weak
 // reference and will not be retained. No-op if |navigation| is null.
 - (void)setState:(web::WKNavigationState)state
     forNavigation:(WKNavigation*)navigation;
 
-// Removes given |navigation|. Fails if |navigation| does not exist. No-op if
-// |navigation| is null.
+// Removes given |navigation|. Fails if |navigation| does not exist.
+// |navigation| can be null.
 - (void)removeNavigation:(WKNavigation*)navigation;
 
 // Adds a new navigation if it was not added yet. If navigation was already
 // added then updates context for existing navigation. Updating context does not
 // affect the result of |lastAddedNavigation| method. |navigation| will be held
 // as a weak reference and will not be retained. No-op if |navigation| is null.
 - (void)setContext:(std::unique_ptr<web::NavigationContextImpl>)context
      forNavigation:(WKNavigation*)navigation;
 
 // Returns context if one was previously associated with given |navigation|.
 // Returns null if |navigation| is null.
 - (web::NavigationContextImpl*)contextForNavigation:(WKNavigation*)navigation;
 
 // WKNavigation which was added the most recently via |setState:forNavigation:|.
 // Updating navigation state via |setState:forNavigation:| does not change the
-// last added navigation. Returns nil if there are no stored navigations.
+// last added navigation. Returns nil if there are no stored navigations or
+// last navigation was null.
 - (WKNavigation*)lastAddedNavigation;
 
 // State of WKNavigation which was added the most recently via
 // |setState:forNavigation:|. WKNavigationState::NONE if CRWWKNavigationStates
 // is empty.
 - (web::WKNavigationState)lastAddedNavigationState;
 
 @end
 
 #endif  // IOS_WEB_WEB_STATE_UI_CRW_WK_NAVIGATION_STATES_H_