Remove all ReadingList entries on managed account signout.

ios/chrome/browser/reading_list/BUILD.gn

 # Copyright 2016 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("reading_list") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "favicon_web_state_dispatcher_impl.h",
     "favicon_web_state_dispatcher_impl.mm",
     "offline_url_utils.cc",
     "offline_url_utils.h",
     "reading_list_distiller_page.h",
     "reading_list_distiller_page.mm",
     "reading_list_distiller_page_factory.h",
     "reading_list_distiller_page_factory.mm",
     "reading_list_download_service.cc",
     "reading_list_download_service.h",
     "reading_list_download_service_factory.cc",
     "reading_list_download_service_factory.h",
     "reading_list_model_factory.cc",
     "reading_list_model_factory.h",
     "reading_list_web_state_observer.h",
     "reading_list_web_state_observer.mm",
     "url_downloader.cc",
     "url_downloader.h",
   ]
   deps = [
+    ":reading_list_remover",
     "//base",
     "//components/browser_sync",
     "//components/dom_distiller/ios",
     "//components/favicon/core",
     "//components/favicon/ios",
     "//components/google/core/browser",
     "//components/keyed_service/core",
     "//components/keyed_service/ios",
     "//components/pref_registry",
     "//components/prefs",
     "//components/reading_list/core",
     "//components/reading_list/ios",
     "//components/sync",
     "//ios/chrome/browser",
     "//ios/chrome/browser/bookmarks",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/favicon",
     "//ios/chrome/browser/history",
     "//ios/chrome/common",
     "//ios/web",
     "//net",
     "//url",
   ]
   public_deps = [
     "//ios/chrome/browser/dom_distiller",
   ]
+  allow_circular_includes_from = [ ":reading_list_remover" ]
+}
+
+source_set("reading_list_remover") {
+  sources = [
+    "reading_list_remover_helper.cc",
+    "reading_list_remover_helper.h",
+  ]
+  deps = [
+    "//base",
+    "//components/reading_list/ios",
+    "//ios/chrome/browser/browser_state",
+  ]
 }
 
 source_set("unit_tests") {
   configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "favicon_web_state_dispatcher_impl_unittest.mm",
     "offline_url_utils_unittest.cc",
     "reading_list_web_state_observer_unittest.mm",
     "url_downloader_unittest.mm",
   ]
   deps = [
     ":reading_list",
     "//base",
     "//base/test:test_support",
     "//components/favicon/ios",
     "//components/reading_list/ios",
     "//ios/chrome/browser",
     "//ios/chrome/browser/browser_state:test_support",
     "//ios/chrome/browser/dom_distiller",
     "//ios/testing:ios_test_support",
     "//ios/web",
     "//ios/web:test_support",
     "//net",
     "//testing/gtest",
     "//url",
   ]
 }