Avoid sending IDC_BACK/FORWARD when back/forward is not possible

ios/chrome/browser/ui/key_commands_provider.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_CHROME_BROWSER_UI_KEY_COMMANDS_PROVIDER_H_
 #define IOS_CHROME_BROWSER_UI_KEY_COMMANDS_PROVIDER_H_
 
 #import <UIKit/UIKit.h>
 
 @protocol KeyCommandsPlumbing<NSObject>
 
 #pragma mark Query information
 
 // Whether the current profile is off-the-record.
 - (BOOL)isOffTheRecord;
 
 // Returns the current number of tabs.
 - (NSUInteger)tabsCount;
 
+// Whether navigation to the previous page is available.
+- (BOOL)canGoBack;
+
+// Whether navigation to the next page is available.
+- (BOOL)canGoForward;
+
 #pragma mark Call for action
 
 // Executes a Chrome command.  |sender| must implement the |-tag| method and
 // return the id of the command to execute.  See UIKit+ChromeExecuteCommand.h
 // for more details.
 - (void)chromeExecuteCommand:(id)sender;
 
 // Called to put the tab at index in focus.
 - (void)focusTabAtIndex:(NSUInteger)index;
 
@@ skipping 14 matching lines @@
 // Handles the keyboard commands registration and handling for the
 // BrowserViewController.
 @interface KeyCommandsProvider : NSObject
 
 - (NSArray*)keyCommandsForConsumer:(id<KeyCommandsPlumbing>)consumer
                        editingText:(BOOL)editingText;
 
 @end
 
 #endif  // IOS_CHROME_BROWSER_UI_KEY_COMMANDS_PROVIDER_H_