[ios clean] Fixes Toolbar navigation crash.

ios/clean/chrome/browser/ui/toolbar/toolbar_coordinator.mm

Index: ios/clean/chrome/browser/ui/toolbar/toolbar_coordinator.mm
diff --git a/ios/clean/chrome/browser/ui/toolbar/toolbar_coordinator.mm b/ios/clean/chrome/browser/ui/toolbar/toolbar_coordinator.mm
index 75be7f58c3af16079f23f586ad29198d7af03742..c6743ce14532c6d0500ccf91b01f02421d291d30 100644
--- a/ios/clean/chrome/browser/ui/toolbar/toolbar_coordinator.mm
+++ b/ios/clean/chrome/browser/ui/toolbar/toolbar_coordinator.mm
@@ -113,11 +113,15 @@
 #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 {