Use GN buildflags and features to configure account consistency

components/signin/core/common/BUILD.gn

 # Copyright 2014 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.
 
+import("//build/buildflag_header.gni")
+import("//components/signin/features.gni")
+
+buildflag_header("signin_features") {
+  header = "signin_features.h"
+  flags = [
+    "ENABLE_DICE_SUPPORT=$enable_dice_support",
+    "ENABLE_MIRROR=$enable_mirror",
+  ]
+}
+
 static_library("common") {
   sources = [
     "profile_management_switches.cc",
     "profile_management_switches.h",
     "signin_pref_names.cc",
     "signin_pref_names.h",
     "signin_switches.cc",
     "signin_switches.h",
   ]
 
   deps = [
     "//base",
   ]
+
+  public_deps = [
+    ":signin_features",
+  ]
 }