[ios] Made IDC_BACK and IDC_FORWARD safe.

ios/chrome/browser/ui/browser_view_controller.mm

Index: ios/chrome/browser/ui/browser_view_controller.mm
diff --git a/ios/chrome/browser/ui/browser_view_controller.mm b/ios/chrome/browser/ui/browser_view_controller.mm
index a612ccb5560b4fadd2c81d1c3d7f9c27a3c55b9d..9263577d11dc9f38eb7e10b983605e59f3252a5b 100644
--- a/ios/chrome/browser/ui/browser_view_controller.mm
+++ b/ios/chrome/browser/ui/browser_view_controller.mm
@@ -3872,7 +3872,10 @@ class BrowserBookmarkModelBridge : public bookmarks::BookmarkModelObserver {
 
   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];
@@ -3913,7 +3916,10 @@ class BrowserBookmarkModelBridge : public bookmarks::BookmarkModelObserver {
       [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();