Split //ios/chrome/browser/prefs to prevent circular dependencies.

ios/chrome/browser/signin/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("signin") {
   sources = [
     "about_signin_internals_factory.cc",
     "about_signin_internals_factory.h",
     "account_consistency_service_factory.h",
     "account_consistency_service_factory.mm",
@@ skipping 102 matching lines @@
     "//components/prefs",
     "//components/signin/core/browser",
     "//components/signin/core/browser:test_support",
     "//components/signin/core/common",
     "//components/signin/ios/browser",
     "//components/sync_preferences",
     "//components/sync_preferences:test_support",
     "//google_apis",
     "//ios/chrome/browser",
     "//ios/chrome/browser/browser_state:test_support",
-    "//ios/chrome/browser/prefs",
+    "//ios/chrome/browser/prefs:browser_prefs",
     "//ios/chrome/browser/sync",
     "//ios/chrome/browser/sync:test_support",
     "//ios/chrome/test:test_support",
     "//ios/public/provider/chrome/browser",
     "//ios/public/provider/chrome/browser/signin",
     "//ios/public/provider/chrome/browser/signin:test_support",
     "//ios/web:test_support",
     "//net:test_support",
     "//testing/gmock",
     "//testing/gtest",
     "//third_party/ocmock",
   ]
 }