Changed previews component to be a layered component.

components/previews/core/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.
 
-static_library("previews") {
+static_library("core") {
   sources = [
     "previews_data_savings.cc",
     "previews_data_savings.h",
     "previews_experiments.cc",
     "previews_experiments.h",
     "previews_io_data.cc",
     "previews_io_data.h",
     "previews_ui_service.cc",
     "previews_ui_service.h",
   ]
 
   deps = [
     "//base",
     "//components/data_reduction_proxy/core/common",
     "//components/variations",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "previews_data_savings_unittest.cc",
     "previews_experiments_unittest.cc",
     "previews_io_data_unittest.cc",
     "previews_ui_service_unittest.cc",
   ]
 
   deps = [
-    ":previews",
+    ":core",
     "//base",
     "//base/test:test_support",
     "//components/data_reduction_proxy/core/common",
     "//components/variations",
     "//testing/gtest",
   ]
 }