media: Implement MediaDrmStorageImpl with tests

components/cdm/browser/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.
 
 source_set("browser") {
   sources = [
     "cdm_message_filter_android.cc",
     "cdm_message_filter_android.h",
     "media_drm_storage_impl.cc",
     "media_drm_storage_impl.h",
   ]
 
   deps = [
     "//base",
     "//components/cdm/common",
     "//components/prefs",
     "//content/public/browser",
     "//content/public/common",
     "//ipc",
     "//media",
     "//media/mojo/interfaces",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "media_drm_storage_impl_unittest.cc",
+  ]
+  deps = [
+    ":browser",
+    "//base/test:test_support",
+    "//components/prefs:test_support",
+    "//media/mojo/services",
+    "//testing/gtest",
+  ]
+}