[iOS] Add flag for the new error page workflow

Bug: 991608
Change-Id: I7f5c8481309bf2f3be8d98586746d7db1e43f4f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1914387
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Reviewed-by: Yi Su <mrsuyi@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715248}

ios/chrome/browser/flags/about_flags.mm

@@ -565,6 +565,9 @@ const flags_ui::FeatureEntry kFeatureEntries[] = {
     {"force-unstacked-tabstrip", flag_descriptions::kForceUnstackedTabstripName,
      flag_descriptions::kForceUnstackedTabstripDescription, flags_ui::kOsIos,
      FEATURE_VALUE_TYPE(kForceUnstackedTabstrip)},
+    {"use-js-error-page", flag_descriptions::kUseJSForErrorPageName,
+     flag_descriptions::kUseJSForErrorPageDescription, flags_ui::kOsIos,
+     FEATURE_VALUE_TYPE(web::features::kUseJSForErrorPage)},
 };
 
 // Add all switches from experimental flags to |command_line|.

ios/chrome/browser/flags/ios_chrome_flag_descriptions.cc

@@ -179,6 +179,10 @@ const char kEnableClipboardProviderTextSuggestionsName[] =
 const char kEnableClipboardProviderTextSuggestionsDescription[] =
     "Enable suggesting a search for text copied to the clipboard";
 
+const char kUseJSForErrorPageName[] = "Enable new error page workflow";
+const char kUseJSForErrorPageDescription[] =
+    "Use JavaScript for the error pages";
+
 const char kEnablePersistentDownloadsName[] = "Enable persistent downloads";
 const char kEnablePersistentDownloadsDescription[] =
     "Enables the new, experimental implementation of persistent downloads";

ios/chrome/browser/flags/ios_chrome_flag_descriptions.h

@@ -149,6 +149,10 @@ extern const char kEnableClipboardProviderImageSuggestionsDescription[];
 extern const char kEnableClipboardProviderTextSuggestionsName[];
 extern const char kEnableClipboardProviderTextSuggestionsDescription[];
 
+// Title and description for the flag to enable the new error page workflow.
+extern const char kUseJSForErrorPageName[];
+extern const char kUseJSForErrorPageDescription[];
+
 // Title and description for the flag to enable persistent downloads.
 extern const char kEnablePersistentDownloadsName[];
 extern const char kEnablePersistentDownloadsDescription[];

ios/web/common/features.h

@@ -48,6 +48,9 @@ extern const base::Feature kLogLoadStartedInDidStartNavigation;
 // Feature flag enabling persistent downloads.
 extern const base::Feature kEnablePersistentDownloads;
 
+// Feature flag for the new error page workflow, using JavaScript.
+extern const base::Feature kUseJSForErrorPage;
+
 // Use WKWebView.loading to update WebState::IsLoading.
 // TODO(crbug.com/1006012): Clean up this flag after experiment.
 bool UseWKWebViewLoading();

ios/web/common/features.mm

@@ -44,6 +44,9 @@ const base::Feature kLogLoadStartedInDidStartNavigation{
 const base::Feature kEnablePersistentDownloads{
     "EnablePersistentDownloads", base::FEATURE_DISABLED_BY_DEFAULT};
 
+const base::Feature kUseJSForErrorPage{"UseJSForErrorPage",
+                                       base::FEATURE_DISABLED_BY_DEFAULT};
+
 // The feature kUseWKWebViewLoading will change the CPM if
 // kLogLoadStartedInDidStartNavigation is not enabled, so
 // kLogLoadStartedInDidStartNavigation is required. The feature flag