Move ContentSuggestions identifiers to custom target

ios/chrome/browser/content_suggestions/BUILD.gn

 # Copyright 2017 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("content_suggestions") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "content_suggestions_category_wrapper.h",
     "content_suggestions_category_wrapper.mm",
     "content_suggestions_coordinator.h",
     "content_suggestions_coordinator.mm",
     "content_suggestions_mediator.h",
     "content_suggestions_mediator.mm",
     "content_suggestions_service_bridge_observer.h",
     "content_suggestions_service_bridge_observer.mm",
   ]
   deps = [
     "//base",
     "//components/ntp_snippets",
     "//ios/chrome/app/strings",
     "//ios/chrome/browser",
     "//ios/chrome/browser/ntp_snippets",
     "//ios/chrome/browser/ui",
     "//ios/chrome/browser/ui/alert_coordinator",
     "//ios/chrome/browser/ui/content_suggestions",
+    "//ios/chrome/browser/ui/content_suggestions/identifier",
     "//ios/web",
     "//ui/base",
     "//ui/strings",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "content_suggestions_category_wrapper_unittest.mm",
   ]
   deps = [
     ":content_suggestions",
     "//components/ntp_snippets",
     "//testing/gtest",
   ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }