[ios] Made IDC_BACK and IDC_FORWARD safe.

ios/chrome/browser/ui/browser_view_controller.mm

 // Copyright 2012 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/chrome/browser/ui/browser_view_controller.h"
 
 #import <AssetsLibrary/AssetsLibrary.h>
 #import <MobileCoreServices/MobileCoreServices.h>
 #import <PassKit/PassKit.h>
 #import <Photos/Photos.h>
@@ skipping 3854 matching lines @@
 #pragma mark - Command Handling
 
 - (IBAction)chromeExecuteCommand:(id)sender {
   NSInteger command = [sender tag];
 
   if (!_model || !_browserState)
     return;
 
   switch (command) {
     case IDC_BACK:
-      [[_model currentTab] goBack];
+      // TODO(crbug.com.677160): Remove |canGoBack| check.
+      if ([_model currentTab].canGoBack) {
+        [[_model currentTab] goBack];
+      }
       break;
     case IDC_BOOKMARK_PAGE:
       [self initializeBookmarkInteractionController];
       [_bookmarkInteractionController
           presentBookmarkForTab:[_model currentTab]
             currentlyBookmarked:_toolbarModelIOS->IsCurrentTabBookmarkedByUser()
                          inView:[_toolbarController bookmarkButtonView]
                      originRect:[_toolbarController bookmarkButtonAnchorRect]];
       break;
     case IDC_CLOSE_TAB:
@@ skipping 20 matching lines @@
             [_findBarController updateResultsCount:model];
           }];
       break;
     case IDC_FIND_CLOSE:
       [self closeFindInPage];
       break;
     case IDC_FIND_UPDATE:
       [self searchFindInPage];
       break;
     case IDC_FORWARD:
-      [[_model currentTab] goForward];
+      // TODO(crbug.com.677160): Remove |canGoForward| check.
+      if ([_model currentTab].canGoForward) {
+        [[_model currentTab] goForward];
+      }
       break;
     case IDC_FULLSCREEN:
       NOTIMPLEMENTED();
       break;
     case IDC_HELP_PAGE_VIA_MENU:
       [self showHelpPage];
       break;
     case IDC_NEW_TAB:
       if (_isOffTheRecord) {
         // Not for this browser state, send it on its way.
@@ skipping 1165 matching lines @@
 
 - (UIView*)voiceSearchButton {
   return _voiceSearchButton;
 }
 
 - (id<LogoAnimationControllerOwner>)logoAnimationControllerOwner {
   return [self currentLogoAnimationControllerOwner];
 }
 
 @end