Add more components_unittests to GN build

components/precache/core/BUILD.gn

 # Copyright 2014 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.
 
 import("//third_party/protobuf/proto_library.gni")
 
 # These values are duplicated in the GYP build in:
 # //components/precache/precache_defines.gypi
 precache_config_settings_url =
     "http://www.gstatic.com/chrome/wifiprefetch/precache_config"
 precache_manifest_url_prefix =
     "http://www.gstatic.com/chrome/wifiprefetch/hosts/"
 
 config("precache_config") {
   defines = [
     "PRECACHE_CONFIG_SETTINGS_URL=\"$precache_config_settings_url\"",
     "PRECACHE_MANIFEST_URL_PREFIX=\"$precache_manifest_url_prefix\"",
   ]
 }
 
-static_library("core") {
+source_set("core") {
   sources = [
     "precache_database.cc",
     "precache_database.h",
     "precache_fetcher.cc",
     "precache_fetcher.h",
     "precache_switches.cc",
     "precache_switches.h",
     "precache_url_table.cc",
     "precache_url_table.h",
   ]
 
   # Note the GYP build sets this as direct dependent settings, but this is
   # only used to share the settings with the unit tests. Instead, we just
   # set this config for the necessary targets manually.
   configs += [ ":precache_config" ]
 
   deps = [
     ":proto",
     "//base",
     "//url",
   ]
 }
 
 proto_library("proto") {
   sources = [
     "proto/precache.proto",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "precache_database_unittest.cc",
+    "precache_fetcher_unittest.cc",
+    "precache_url_table_unittest.cc",
+  ]
+  deps = [
+    ":core",
+    ":proto",
+    "//testing/gmock",
+    "//testing/gtest",
+  ]
+}