Reland "Enable iOS Captive Portal feature by default."

Failing tests listed in crrev.com/c/778283 were fixed by
crrev.com/c/780363

This is a reland of beecf799
Original change's description:
> Enable iOS Captive Portal feature by default.
>
> Bug: 750228
> Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
> Change-Id: I0fa138e855d71ac256759d75f3e84d390cd2c896
> Reviewed-on: https://chromium-review.googlesource.com/777468
> Reviewed-by: Eugene But <eugenebut@chromium.org>
> Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#517579}

Bug: 750228
Change-Id: Idb7aff0f30b9f5cf1f3317355442a34598cc1893
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Reviewed-on: https://chromium-review.googlesource.com/798643Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520580}

ios/chrome/browser/ssl/captive_portal_features.cc

@@ -5,4 +5,4 @@
 #include "ios/chrome/browser/ssl/captive_portal_features.h"
 
 const base::Feature kCaptivePortalFeature{"CaptivePortalFeature",
-                                          base::FEATURE_DISABLED_BY_DEFAULT};
+                                          base::FEATURE_ENABLED_BY_DEFAULT};