Make DETECT_IMPORTANT_CONTENT the default Plugins content setting based

components/content_settings/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.
 
 static_library("common") {
   sources = [
     "content_settings.cc",
     "content_settings.h",
     "content_settings_pattern.cc",
     "content_settings_pattern.h",
     "content_settings_pattern_parser.cc",
     "content_settings_pattern_parser.h",
+    "content_settings_switches.cc",
+    "content_settings_switches.h",
     "content_settings_types.h",
     "permission_request_id.cc",
     "permission_request_id.h",
     "pref_names.cc",
     "pref_names.h",
   ]
 
   deps = [
     "//base",
     "//net",
     "//url",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "content_settings_pattern_parser_unittest.cc",
     "content_settings_pattern_unittest.cc",
   ]
 
   deps = [
     ":common",
     "//net",
     "//testing/gmock",
     "//testing/gtest",
     "//url",
   ]
 }