Enabled WebErrorPages feature by default.

No major issues were found during manual testing.
Safe to ship in M69.

Bug: 725241
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I753c20d6525547958140d6407e8bd71d4f8ef0f7
Reviewed-on: https://chromium-review.googlesource.com/1086111Reviewed-by: Danyao Wang <danyao@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569024}

ios/web/features.mm

@@ -20,7 +20,7 @@ const base::Feature kNewFileDownload{"NewFileDownload",
                                      base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kWebErrorPages{"WebErrorPages",
-                                   base::FEATURE_DISABLED_BY_DEFAULT};
+                                   base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kWKHTTPSystemCookieStore{"WKHTTPSystemCookieStore",
                                              base::FEATURE_DISABLED_BY_DEFAULT};