[Popular Sites] Move PopularSitesImpl to dedicated file

components/ntp_tiles/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.
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
 static_library("ntp_tiles") {
   sources = [
     "constants.cc",
     "constants.h",
     "country_code_ios.h",
     "country_code_ios.mm",
     "field_trial.cc",
     "field_trial.h",
     "icon_cacher.cc",
     "icon_cacher.h",
     "metrics.cc",
     "metrics.h",
     "most_visited_sites.cc",
     "most_visited_sites.h",
     "ntp_tile.cc",
     "ntp_tile.h",
     "ntp_tile_source.h",
-    "popular_sites.cc",
     "popular_sites.h",
+    "popular_sites_impl.cc",
+    "popular_sites_impl.h",
     "pref_names.cc",
     "pref_names.h",
     "switches.cc",
     "switches.h",
     "webui/ntp_tiles_internals_message_handler.cc",
     "webui/ntp_tiles_internals_message_handler.h",
     "webui/ntp_tiles_internals_message_handler_client.h",
     "webui/popular_sites_internals_message_handler.cc",
     "webui/popular_sites_internals_message_handler.h",
     "webui/popular_sites_internals_message_handler_client.h",
@@ skipping 39 matching lines @@
     "//base",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "icon_cacher_unittest.cc",
     "metrics_unittest.cc",
     "most_visited_sites_unittest.cc",
-    "popular_sites_unittest.cc",
+    "popular_sites_impl_unittest.cc",
   ]
 
   deps = [
     ":json_unsafe_parser",
     ":ntp_tiles",
     "//base/test:test_support",
     "//components/favicon/core",
     "//components/favicon_base",
     "//components/image_fetcher",
     "//components/pref_registry:pref_registry",
     "//components/rappor:test_support",
     "//components/sync_preferences:test_support",
     "//net:test_support",
     "//testing/gmock",
     "//testing/gtest",
     "//ui/gfx:test_support",
   ]
 }
 
 if (is_android) {
   java_cpp_enum("ntp_tiles_enums_java") {
     sources = [
       "metrics.h",
       "ntp_tile_source.h",
     ]
   }
 }