Initialize ios/web_view translate with a system-wide URLRequestContext.

ios/components/io_thread/BUILD.gn

+# Copyright 2017 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.
+
+component("io_thread") {
+  configs += [ "//build/config/compiler:enable_arc" ]
+
+  deps = [
+    "//base",
+    "//components/net_log",
+    "//components/network_session_configurator",
+    "//components/prefs",
+    "//components/proxy_config",
+    "//components/proxy_config/ios",
+    "//components/ssl_config",
+    "//components/variations",
+    "//components/version_info",
+    "//ios/web",
+    "//ios/web:user_agent",
+    "//net",
+  ]
+
+  sources = [
+    "io_thread.h",
+    "io_thread.mm",
+  ]
+}