Remove the 'ios_use_webrtc' build flag hack.

ios/public/provider/chrome/browser/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 source_set("browser") {
   sources = [
     "chrome_browser_provider.h",
     "chrome_browser_provider.mm",
     "geolocation_updater_provider.h",
     "geolocation_updater_provider.mm",
     "signin/chrome_identity.h",
     "signin/chrome_identity.mm",
     "signin/chrome_identity_service.h",
     "signin/chrome_identity_service.mm",
     "signin/signin_error_provider.h",
     "signin/signin_error_provider.mm",
-    "string_provider.cc",
-    "string_provider.h",
     "ui/default_ios_web_view_factory.h",
     "ui/default_ios_web_view_factory.mm",
     "ui/infobar_view_delegate.h",
     "ui/infobar_view_protocol.h",
     "updatable_resource_provider.h",
     "updatable_resource_provider.mm",
   ]
 
   deps = [
     "//base",
@@ skipping 15 matching lines @@
     "test_updatable_resource_provider.mm",
   ]
 
   deps = [
     ":browser",
     "//base",
     "//components/signin/ios/browser:test_support",
     "//testing/gtest",
   ]
 }