[ios] Disable offline mode fix for SlimNav on iOS13.

On iOS 12 airplane mode back/forward navigations between restored session
entries are same-document navigattions and were not triggering onload()
or onpopstate() redirect to the actual target URL.  As a workaround
a reload is triggered.

It appears this work-around isn't necessary on iOS13.

TEST=Visit techmeme, wikipedia + a few articles, tab grid, close all, restore, airplane mode, back to tab, tap back to beginning.

Bug: 995363
Change-Id: I0fcd6ad8ce5172b4808030c4cee0166289e6e22c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1757298
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Danyao Wang <danyao@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688331}

ios/web/navigation/crw_web_view_navigation_observer.mm

@@ -144,19 +144,23 @@ using web::wk_navigation_util::IsPlaceholderUrl;
   // not fire 'pageshow', 'onload', 'popstate' or any of the
   // WKNavigationDelegate callbacks for back/forward navigation from an about:
   // scheme placeholder URL to another entry or if either of the redirect fails
-  // to load (e.g. in airplane mode). Loading state KVO is the only observable
-  // event in this scenario, so force a reload to trigger redirect from
-  // restore_session.html to the restored URL.
+  // to load (e.g. in airplane mode, <iOS13). Loading state KVO is the only
+  // observable event in this scenario, so force a reload to trigger redirect
+  // from restore_session.html to the restored URL.
   bool previousURLHasAboutScheme =
       self.documentURL.SchemeIs(url::kAboutScheme) ||
       IsPlaceholderUrl(self.documentURL) ||
       web::GetWebClient()->IsAppSpecificURL(self.documentURL);
-  bool is_back_forward_navigation =
+  bool needs_back_forward_navigation_reload =
       existingContext &&
       (existingContext->GetPageTransition() & ui::PAGE_TRANSITION_FORWARD_BACK);
+  // The back-forward workaround isn't need on iOS13.
+  if (@available(iOS 13, *)) {
+    needs_back_forward_navigation_reload = false;
+  }
   if (web::GetWebClient()->IsSlimNavigationManagerEnabled() &&
       IsRestoreSessionUrl(webViewURL)) {
-    if (previousURLHasAboutScheme || is_back_forward_navigation) {
+    if (previousURLHasAboutScheme || needs_back_forward_navigation_reload) {
       [self.webView reload];
       self.navigationHandler.navigationState =
           web::WKNavigationState::REQUESTED;