Migrate ArcBridgeService::Observer and ArcSession::Observer.

components/arc/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.
 
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//testing/test.gni")
 
 static_library("arc") {
   sources = [
     "arc_bridge_host_impl.cc",
@@ skipping 86 matching lines @@
     "//url:url",
   ]
 }
 
 static_library("arc_base") {
   sources = [
     "arc_bridge_service.cc",
     "arc_bridge_service.h",
     "arc_service.cc",
     "arc_service.h",
+    "arc_session_observer.cc",
+    "arc_session_observer.h",
     "instance_holder.h",
   ]
 
   deps = [
     "//base",
     "//chromeos",
   ]
 
   public_deps = [
     ":arc_bindings",
@@ skipping 109 matching lines @@
     "//testing/gtest",
     "//ui/aura",
     "//ui/aura:test_support",
     "//ui/base:test_support",
     "//ui/base/ime",
     "//ui/events",
     "//ui/events:dom_keycode_converter",
     "//url:url",
   ]
 }