[DevTools] Migrate from AgentStateCallbacks to DevToolsAgentHostObserver.

content/public/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.
 
 import("//build/config/ui.gni")
 
 # See //content/BUILD.gn for how this works.
 group("browser") {
   if (is_component_build) {
     public_deps = [
@@ skipping 69 matching lines @@
     "color_chooser.h",
     "content_browser_client.cc",
     "content_browser_client.h",
     "context_factory.h",
     "cookie_store_factory.h",
     "desktop_media_id.cc",
     "desktop_media_id.h",
     "desktop_notification_delegate.h",
     "devtools_agent_host.h",
     "devtools_agent_host_client.h",
+    "devtools_agent_host_observer.h",
     "devtools_external_agent_proxy.h",
     "devtools_external_agent_proxy_delegate.h",
     "devtools_frontend_host.h",
     "devtools_manager_delegate.cc",
     "devtools_manager_delegate.h",
     "devtools_socket_factory.h",
     "dom_storage_context.h",
     "download_danger_type.h",
     "download_interrupt_reason_values.h",
     "download_interrupt_reasons.h",
@@ skipping 215 matching lines @@
 
   if (is_android) {
     deps += [ "//ui/android" ]
   }
 
   if (use_aura) {
     sources -= [ "context_factory.h" ]
     deps += [ "//ui/aura" ]
   }
 }