[ios] Try to avoid crash on null context during navigation

Add a missing null check on context.
Also changed navigation not to check state to determine if commit has
already happened, since the state machine being uncertain, state not
being 'commit' doesn't mean commit has not been called before.

Bug: 1041368
Change-Id: I6f77c2137e30c46b8180f35e584833cd57f8f7db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2003134Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: David Jean <djean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733568}

ios/web/navigation/crw_wk_navigation_handler.mm

@@ -757,7 +757,7 @@ void ReportOutOfSyncURLInDidStartProvisionalNavigation(
 
   web::NavigationContextImpl* context =
       [self.navigationStates contextForNavigation:navigation];
-  if (!web::IsWKWebViewSSLCertError(context->GetError())) {
+  if (context && !web::IsWKWebViewSSLCertError(context->GetError())) {
     _certVerificationErrors->Clear();
   }
 
@@ -945,8 +945,7 @@ void ReportOutOfSyncURLInDidStartProvisionalNavigation(
   // Sometimes |webView:didFinishNavigation| arrives before
   // |webView:didCommitNavigation|. Explicitly trigger post-commit processing.
   bool navigationCommitted =
-      [self.navigationStates stateForNavigation:navigation] ==
-      web::WKNavigationState::COMMITTED;
+      [self.navigationStates isCommittedNavigation:navigation];
   UMA_HISTOGRAM_BOOLEAN("IOS.WKWebViewFinishBeforeCommit",
                         !navigationCommitted);
   if (!navigationCommitted) {