[ios/web] Don't overwrite virtual url in didStartProvisionalNavigation.

When correcting the NavigationItem URL, such as a restore targetURL,
don't reset the visible URL. Doing so breaks navigating back to an NTP
by overwriting the NTP's visible URL.

Change-Id: If220cf65ff5bebae72e7a7e42c5ba94beab3e170
Reviewed-on: https://chromium-review.googlesource.com/c/1490056
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Auto-Submit: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635880}

ios/web/web_state/ui/crw_web_controller.mm

@@ -4800,7 +4800,6 @@ GURL URLEscapedForHistory(const GURL& url) {
           self.navigationManagerImpl, context->GetNavigationItemUniqueID());
       if (!web::wk_navigation_util::IsWKInternalUrl(webViewURL)) {
         if (item) {
-          item->SetVirtualURL(webViewURL);
           item->SetURL(webViewURL);
         }
         context->SetUrl(webViewURL);