Fixing WKWebView QuickBack

ios/web/web_state/ui/crw_wk_web_view_web_controller.mm

Index: ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
diff --git a/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm b/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
index 0d1845039df51e0c8d9b331d76f3f73df2746cab..94797002ebab74f59c0a800b4abdd075398c690c 100644
--- a/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
+++ b/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
@@ -449,19 +449,17 @@ WKWebViewErrorSource WKWebViewErrorSourceFromError(NSError* error) {
     [self loadRequest:[self requestForCurrentNavigationItem]];
   };
 
-  // If there is no corresponding WKBackForwardListItem, fall back to
-  // the standard loading mechanism.
+  // If there is no corresponding WKBackForwardListItem, or the item is not in
+  // the current WKWebView's back-forward list, we can only fall back to the

[Eugene But (OOO till 7-30), 2015/09/14 21:48:55]

NIT: please do not use we in the comments.

+  // default navigation mechanism.
   web::WKBackForwardListItemHolder* holder =
       [self currentBackForwardListItemHolder];
-  if (!holder->back_forward_list_item()) {
+  if (!holder->back_forward_list_item() ||
+      ![self isBackForwardListItemValid:holder->back_forward_list_item()]) {
     defaultNavigationBlock();
     return;
   }
 
-  // The current back-forward list item MUST be in the WKWebView's back-forward
-  // list to be valid.
-  DCHECK([self isBackForwardListItemValid:holder->back_forward_list_item()]);
-
   ProceduralBlock webViewNavigationBlock = ^{
     // If the current navigation URL is the same as the URL of the visible
     // page, that means the user requested a reload. |goToBackForwardListItem|