[iOS] Be more cautious with pending navigation when cancelling load

When a page load is cancelled, the pending navigation shouldn't
necessarly be discarded because another navigation could be the current
pending navigation (if there is a delay).
This CL makes sure to check that the cancelled navigation is the
pending navigation before discarding the pending navigation.

Bug: 1007243
Change-Id: Iae01e31bea3e40ae4ff08f1dbcc4a28c9f70198c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1837931
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702874}

ios/web/navigation/crw_wk_navigation_handler.mm

@@ -1788,7 +1788,11 @@ - (void)handleCancelledError:(NSError*)error
     std::unique_ptr<web::NavigationContextImpl> navigationContext =
         [self.navigationStates removeNavigation:navigation];
     [self loadCancelled];
-    self.navigationManagerImpl->DiscardNonCommittedItems();
+    web::NavigationItemImpl* item = web::GetItemWithUniqueID(
+        self.navigationManagerImpl, navigationContext.get());
+    if (self.navigationManagerImpl->GetPendingItem() == item) {
+      self.navigationManagerImpl->DiscardNonCommittedItems();
+    }
 
     [self.legacyNativeContentController
         handleCancelledErrorForContext:navigationContext.get()];