GN: Add assert(is_chromeos) to //chrome/browser/chromeos

chrome/browser/chromeos/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/config/features.gni")
 import("//build/config/ui.gni")
 import("//chrome/chrome_tests.gni")
 import("//media/media_options.gni")
 import("//third_party/protobuf/proto_library.gni")
 
+assert(is_chromeos)
+
 gypi_values = exec_script("//build/gypi_to_gn.py",
                           [ rebase_path("../../chrome_browser_chromeos.gypi") ],
                           "scope",
                           [ "../../chrome_browser_chromeos.gypi" ])
 
 source_set("chromeos") {
   assert(enable_extensions, "ChromeOS Chrome has to be built with extensions")
 
   configs += [ "//build/config/compiler:wexit_time_destructors" ]
 
@@ skipping 175 matching lines @@
     "policy/proto/install_attributes.proto",
   ]
 }
 
 proto_library("attestation_proto") {
   sources = [
     "attestation/attestation_key_payload.proto",
     "attestation/attestation_signed_data.proto",
   ]
 }