[iOS] Early return if the navigation context is nil during error

When failing a provisional navigation, the context might be nil
(probably a race condition in the callbacks of the WKWebView).
This issue was handled in handleCancelledError:forNavigation:
provisionalLoad: but wasn't in the other callbacks.
This CL makes sure that we handle it.
The workaround in crrev.com/c/1700054 was probably preventing the
crash from happening.

Bug: 1004634
Change-Id: I1f94812617c908a63124c16a18a3ca71d9902d8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1816575
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699741}

ios/web/navigation/crw_wk_navigation_handler.mm

@@ -1636,6 +1636,22 @@ void ReportOutOfSyncURLInDidStartProvisionalNavigation(
 
   web::NavigationContextImpl* navigationContext =
       [self.navigationStates contextForNavigation:navigation];
+
+  if (@available(iOS 13, *)) {
+  } else {
+    if (provisionalLoad && !navigationContext &&
+        web::RequiresProvisionalNavigationFailureWorkaround()) {
+      // It is likely that |navigationContext| is null because
+      // didStartProvisionalNavigation: was not called with this WKNavigation
+      // object. Log UMA to know when this workaround can be removed and
+      // do not call OnNavigationFinished() to avoid crash on null pointer
+      // dereferencing. See crbug.com/973653 and crbug.com/1004634 for details.
+      UMA_HISTOGRAM_BOOLEAN(
+          "Navigation.IOSNullContextInDidFailProvisionalNavigation", true);
+      return;
+    }
+  }
+
   navigationContext->SetError(error);
   navigationContext->SetIsPost([self isCurrentNavigationItemPOST]);
   // TODO(crbug.com/803631) DCHECK that self.currentNavItem is the navigation