[Doodle] Split DoodleFetcher into interface + Impl

components/doodle/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.
 
 static_library("doodle") {
   sources = [
-    "doodle_fetcher.cc",
     "doodle_fetcher.h",
+    "doodle_fetcher_impl.cc",
+    "doodle_fetcher_impl.h",
     "doodle_types.h",
   ]
 
   deps = [
     "//base",
     "//components/data_use_measurement/core",
     "//components/google/core/browser",
     "//net",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
-    "doodle_fetcher_unittest.cc",
+    "doodle_fetcher_impl_unittest.cc",
   ]
 
   deps = [
     ":doodle",
     "//components/google/core/browser",
     "//net:test_support",
     "//ui/base",
   ]
 }