[ios clean] Fixes Toolbar navigation crash.

ios/clean/chrome/browser/ui/toolbar/toolbar_coordinator.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/clean/chrome/browser/ui/toolbar/toolbar_coordinator.h"
 
 #import "ios/clean/chrome/browser/ui/commands/tools_menu_commands.h"
 #import "ios/clean/chrome/browser/ui/omnibox/location_bar_coordinator.h"
 #import "ios/clean/chrome/browser/ui/toolbar/toolbar_mediator.h"
 #import "ios/clean/chrome/browser/ui/toolbar/toolbar_view_controller.h"
@@ skipping 95 matching lines @@
 
 - (void)closeToolsMenu {
   [self.toolsMenuCoordinator stop];
   [self removeChildCoordinator:self.toolsMenuCoordinator];
 }
 
 // PLACEHOLDER: These will move to an object that handles navigation.
 #pragma mark - NavigationCommands
 
 - (void)goBack {
-  self.webState->GetNavigationManager()->GoBack();
+  if (self.webState->GetNavigationManager()->CanGoBack()) {
+    self.webState->GetNavigationManager()->GoBack();

[edchin, 2017/04/10 05:13:15]

I wonder if the GoBack() method should simply no-op if you can't go back. 

[marq (ping after 24h), 2017/04/10 11:14:09]

If it's a programming error to call GoBack() when CanGoBack() is false, then the
public interface for those functions should say as much, and it should be
enforced by DCHECK.

If it's not a programming error, then it should silently no-op.

Ask eugenebut@ which approach he would prefer. 

[rohitrao (ping after 24h), 2017/04/10 12:47:03]

I would prefer that GoBack() fail loudly if CanGoBack() returns false.  This is
kind of area where silently doing nothing is more likely to cause bugs than
DCHECKing.  (As another point in favor, //content also has a NOTREACHED() in
this codepath.)

Another option would be to DCHECK(CanGoBack()) here and make sure that callers
don't invoke this command.  That's probably not a good idea, because UI is
weird.  But it would be nice to understand exactly why it's possible to mash
fast enough to call this command multiple times before the button updates and
gets disabled.

+  }
 }
 
 - (void)goForward {
-  self.webState->GetNavigationManager()->GoForward();
+  if (self.webState->GetNavigationManager()->CanGoForward()) {
+    self.webState->GetNavigationManager()->GoForward();
+  }
 }
 
 - (void)stopLoadingPage {
   self.webState->Stop();
 }
 
 - (void)reloadPage {
   self.webState->GetNavigationManager()->Reload(web::ReloadType::NORMAL,
                                                 false /*check_for_repost*/);
 }
 
 @end