Make MediaSession a runtime-enabled feature on Desktop.

content/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/features.gni")
 import("//build/config/ui.gni")
 import("//content/browser/browser.gni")
 import("//media/media_options.gni")
 import("//mojo/public/mojo_application_manifest.gni")
 
@@ skipping 338 matching lines @@
             content_browser_gypi_values.android_in_process_browser_sources,
             ".",
             "//content")
     sources -= [
       "browser_ipc_logging.cc",
       "device_sensors/data_fetcher_shared_memory_default.cc",
       "geolocation/network_location_provider.cc",
       "geolocation/network_location_provider.h",
       "geolocation/network_location_request.cc",
       "geolocation/network_location_request.h",
+      "media/session/media_session_delegate_default.cc",
       "power_usage_monitor_impl.cc",
       "power_usage_monitor_impl.h",
       "renderer_host/begin_frame_observer_proxy.cc",
       "tracing/tracing_ui.cc",
       "tracing/tracing_ui.h",
 
       # Android skips most, but not all, of the speech code.
       "speech/audio_buffer.cc",
       "speech/audio_buffer.h",
       "speech/audio_encoder.cc",
@@ skipping 170 matching lines @@
       "vr/android/cardboard/cardboard_vr_device_provider.h",
     ]
   }
 }
 
 mojo_application_manifest("chrome_renderer_manifest") {
   type = "exe"
   application_name = "chrome_renderer"
   source = "mojo/chrome_renderer_manifest.json"
 }