Add a ServiceWorkerContextObserver to content's public API.

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 209 matching lines @@
     "resource_request_info.h",
     "resource_throttle.cc",
     "resource_throttle.h",
     "restore_type.h",
     "save_page_type.h",
     "screen_orientation_delegate.h",
     "security_style_explanation.h",
     "security_style_explanations.cc",
     "security_style_explanations.h",
     "service_worker_context.h",
+    "service_worker_context_observer.h",
     "service_worker_usage_info.cc",
     "service_worker_usage_info.h",
     "session_storage_namespace.h",
     "session_storage_usage_info.h",
     "site_instance.h",
     "speech_recognition_event_listener.h",
     "speech_recognition_manager.h",
     "speech_recognition_manager_delegate.h",
     "speech_recognition_session_config.cc",
     "speech_recognition_session_config.h",
@@ skipping 108 matching lines @@
     ]
   }
 
   if (is_chromeos) {
     sources += [
       "arc_tracing_agent.cc",
       "arc_tracing_agent.h",
     ]
   }
 }