[Nav Experiment] Early exit if CommitPendingItem is already called.

CommitPendingItem may be called more than once. For example, when a
navigation fails after it is committed, |webView:didFailNavigation|
calls |loadNativeViewWithSuccess|, which calls CommitPendingItem a
second time even though it was already called once in
|webView:didCommitNavigation|.

NavigationAndLoadCallbacksTest.FailedLoad is failing because of this
reason. This CL fixes the test and the occasional crash observed in
WKBasedNavigationManagerImpl::CommitPendingItem.

Bug: 795475
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I62bbd7b87e26c4e82f197d533a8d5951cf2531e0
Reviewed-on: https://chromium-review.googlesource.com/1002893Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Danyao Wang <danyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549290}

ios/web/navigation/wk_based_navigation_manager_impl.mm

@@ -159,6 +159,11 @@ void WKBasedNavigationManagerImpl::AddPendingItem(
 }
 
 void WKBasedNavigationManagerImpl::CommitPendingItem() {
+  // CommitPendingItem may be called multiple times. Do nothing if there is no
+  // pending item.
+  if (pending_item_index_ == -1 && !pending_item_)
+    return;
+
   bool last_committed_item_was_empty_window_open_item =
       empty_window_open_item_ != nullptr;
 
@@ -190,6 +195,7 @@ void WKBasedNavigationManagerImpl::CommitPendingItem() {
   }
 
   pending_item_index_ = -1;
+  pending_item_.reset();
   // If the last committed item is the empty window open item, then don't update
   // previous item because the new commit replaces the last committed item.
   if (!last_committed_item_was_empty_window_open_item) {