Move ContentSuggestions identifiers to custom target

ios/chrome/browser/ui/content_suggestions/identifier/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("identifier") {
+  configs += [ "//build/config/compiler:enable_arc" ]
+  sources = [
+    "content_suggestion_identifier.h",
+    "content_suggestion_identifier.mm",
+    "content_suggestions_section_information.h",
+    "content_suggestions_section_information.mm",
+  ]
+  deps = [
+    "//base",
+  ]
+}
+
+source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
+  testonly = true
+  sources = [
+    "content_suggestion_identifier_unittest.mm",
+  ]
+  deps = [
+    ":identifier",
+    "//testing/gtest",
+  ]
+}