[ios] Adds a helper function to register prefs for Physical Web.

ios/chrome/browser/physical_web/BUILD.gn

 # Copyright 2016 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("physical_web") {
   sources = [
     "create_physical_web_data_source.h",
     "create_physical_web_data_source.mm",
     "ios_chrome_physical_web_data_source.h",
     "ios_chrome_physical_web_data_source.mm",
     "physical_web_constants.h",
+    "physical_web_prefs_registration.cc",
+    "physical_web_prefs_registration.h",
     "start_physical_web_discovery.h",
     "start_physical_web_discovery.mm",
   ]
   deps = [
     "//base",
     "//components/physical_web/data_source",
+    "//components/pref_registry",
     "//components/prefs",
     "//ios/chrome/browser",
     "//ios/chrome/browser/geolocation",
     "//ios/chrome/common/physical_web",
     "//url",
   ]
   libs = [ "CoreLocation.framework" ]
 }