Move GN build files for cacheinvalidation into the main tree.

third_party/cacheinvalidation/src/google/cacheinvalidation/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("//third_party/protobuf/proto_library.gni")
 
 proto_library("cacheinvalidation_proto_cpp") {
   # Depend on cacheinvalidation instead.
   visibility = [ "//third_party/cacheinvalidation/*" ]
 
   sources = [
     "client.proto",
     "client_gateway.proto",
     "client_protocol.proto",
     "client_test_internal.proto",
     "types.proto",
   ]
 
   if (!is_android) {
     sources += [
       "android_channel.proto",
       "channel_common.proto",
     ]
   }
 
   proto_out_dir = "google/cacheinvalidation"
 }