[Offline] Creates metadata store interface for offline pages

components/offline_pages/BUILD.gn

 # Copyright 2015 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.
 
-# GYP: //components/offline_pages.gypi:offline_pages'
+import("//third_party/protobuf/proto_library.gni")
+
+# GYP: //components/offline_pages.gypi:offline_pages
 static_library("offline_pages") {
   sources = [
     "offline_page_item.cc",
     "offline_page_item.h",
+    "offline_page_metadata_store.cc",
+    "offline_page_metadata_store.h",
+    "offline_page_metadata_store_impl.cc",
+    "offline_page_metadata_store_impl.h",
     "offline_page_model.cc",
     "offline_page_model.h",
   ]
 
   deps = [
+    ":proto",
     "//base",
     "//components/keyed_service/core",
     "//net",
+    "//third_party/leveldatabase",
+    "//third_party/protobuf:protobuf_lite",
     "//url",
   ]
 }
+
+proto_library("proto") {
+  sources = [
+    "proto/offline_pages.proto",
+  ]
+}
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "offline_page_metadata_store_impl_unittest.cc",
+  ]
+
+  deps = [
+    ":offline_pages",
+    "//testing/gtest",
+  ]
+}