Allow retrieval of media device ID salt even after ResourceContext has gone away.

TBR=willchan@chromium.org
BUG=341211

Review URL: https://codereview.chromium.org/143003031

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251024 0039d316-1c4b-4281-b951-d872f2087c98

• chrome/browser/extensions/api/webrtc_audio_private/webrtc_audio_private_api.cc[diff]
• chrome/browser/extensions/api/webrtc_audio_private/webrtc_audio_private_browsertest.cc[diff]
• chrome/browser/media/media_device_id_salt.h[diff]
• chrome/browser/profiles/profile_io_data.cc[diff]
• chrome/browser/profiles/profile_io_data.h[diff]
• content/browser/loader/resource_scheduler_unittest.cc[diff]
• content/browser/renderer_host/media/device_request_message_filter.cc[diff]
• content/browser/renderer_host/media/device_request_message_filter_unittest.cc[diff]
• content/browser/renderer_host/media/media_stream_dispatcher_host.cc[diff]
• content/browser/renderer_host/media/media_stream_dispatcher_host_unittest.cc[diff]
• content/browser/renderer_host/media/media_stream_manager.cc[diff]
• content/browser/renderer_host/media/media_stream_manager.h[diff]
• content/browser/renderer_host/media/video_capture_host_unittest.cc[diff]
• content/browser/resource_context_impl.cc[diff]
• content/public/browser/media_device_id.cc[diff]
• content/public/browser/media_device_id.h[diff]
• content/public/browser/resource_context.h[diff]
• content/test/webrtc_audio_device_test.cc[diff]