Do not call PolicyDecider callbacks for placeholder URLs.

Placeholder URLs are also used for WebErrorPages features, so this CL
ignores placeholder URLs in decidePolicy callbacks if WebErrorPages is
enabled.

Bug: 725241
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I54fb0a5e6dd692d741db4bac298f760165d276f7
Reviewed-on: https://chromium-review.googlesource.com/1087364Reviewed-by: Danyao Wang <danyao@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564652}

ios/web/web_state/ui/crw_web_controller.mm

@@ -4225,13 +4225,16 @@ registerLoadRequestForURL:(const GURL&)requestURL
   // retrieved state will be pending until |didCommitNavigation| callback.
   [self updatePendingNavigationInfoFromNavigationAction:action];
 
-  if (web::GetWebClient()->IsSlimNavigationManagerEnabled()) {
+  if (web::GetWebClient()->IsSlimNavigationManagerEnabled() ||
+      base::FeatureList::IsEnabled(web::features::kWebErrorPages)) {
     // If this is a placeholder navigation, pass through.
     if (IsPlaceholderUrl(requestURL)) {
       decisionHandler(WKNavigationActionPolicyAllow);
       return;
     }
+  }
 
+  if (web::GetWebClient()->IsSlimNavigationManagerEnabled()) {
     // WKBasedNavigationManager doesn't use |loadCurrentURL| for reload or back/
     // forward navigation. So this is the first point where a form repost would
     // be detected. Display the confirmation dialog.
@@ -4305,7 +4308,8 @@ registerLoadRequestForURL:(const GURL&)requestURL
   GURL responseURL = net::GURLWithNSURL(navigationResponse.response.URL);
 
   // If this is a placeholder navigation, pass through.
-  if (web::GetWebClient()->IsSlimNavigationManagerEnabled() &&
+  if ((web::GetWebClient()->IsSlimNavigationManagerEnabled() ||
+       base::FeatureList::IsEnabled(web::features::kWebErrorPages)) &&
       IsPlaceholderUrl(responseURL)) {
     handler(WKNavigationResponsePolicyAllow);
     return;