[Offline pages] Introduces TaskQueue to serialize tasks that asynchronously access SQLStore

components/offline_pages/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.
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
-static_library("request_header") {
+static_library("core") {
   sources = [
-    "offline_page_header.cc",
-    "offline_page_header.h",
+    "task.cc",
+    "task.h",
+    "task_queue.cc",
+    "task_queue.h",
   ]
 
   deps = [
     "//base",
   ]
 }
 
+static_library("test_support") {
+  testonly = true
+  sources = [
+    "test_task.cc",
+    "test_task.h",
+  ]
+
+  deps = [
+    ":core",
+    "//base",
+  ]
+}
+
 source_set("unit_tests") {
   testonly = true
   sources = [
-    "offline_page_header_unittest.cc",
+    "task_queue_unittest.cc",
+    "task_unittest.cc",
   ]
 
   deps = [
-    ":request_header",
+    ":core",
+    ":test_support",
     "//base",
     "//base/test:test_support",
-    "//testing/gtest",
+    "//testing/gtest:gtest",
   ]
 }