guidou@: I am somewhat familiar with this part of the code. If you have not begun working on this, can I take up this task? I myself was looking to cut down few fields from MediaStreamDevice as much as possible, to simplify typemapping for this Struct.

Yes, you can take it. 

guidou@: The |input| field, being a AudioParameter has to be valid for it to be sent over IPC/mojo. So, making it base::Optional instead of using UnvailableDeviceParams() be appropriate? How do we ensure that |input| is valid?

For |input|, the change is basically to replace UnvailableDeviceParams() with no value (nullopt). nullopt is serializable.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c

commit 90ebce39c2a621d47c5e3c8d84bc140d01d2f67c
Author: Chandan Padhi <c.padhi@samsung.com>
Date: Fri Dec 15 03:56:44 2017

Remove |matched_output| field from content::MediaStreamDevice

Instead, |matched_output_device_id| will be used to fetch output audio parameters
for the associated output device or to decide if remote audio tracks should be
rendered to the associated output device.

Bug: 793255
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ic2954a6f6e99597c70b26abfb15a6c69d65b848a
Reviewed-on: https://chromium-review.googlesource.com/822311
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Max Morin <maxmorin@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Chandan Padhi (OOO Fri) <c.padhi@samsung.com>
Cr-Commit-Position: refs/heads/master@{#524315}
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/browser/renderer_host/media/audio_input_device_manager.cc
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/browser/renderer_host/media/audio_input_device_manager.h
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/browser/renderer_host/media/audio_input_device_manager_unittest.cc
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/browser/renderer_host/media/audio_output_authorization_handler.cc
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/browser/renderer_host/media/audio_output_authorization_handler.h
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/browser/renderer_host/media/media_stream_manager.cc
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/common/media/media_stream_param_traits.h
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/public/common/media_stream_request.h
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/renderer/media/media_stream_renderer_factory_impl.cc
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/renderer/media/mock_mojo_media_stream_dispatcher_host.cc
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/renderer/media/user_media_client_impl_unittest.cc
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/renderer/media/user_media_processor.cc
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/renderer/media/webrtc/processed_local_audio_source.cc
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/renderer/media/webrtc_audio_device_impl.cc
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/content/renderer/media/webrtc_audio_device_impl.h
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/media/audio/audio_system.h
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/media/audio/audio_system_helper.cc
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/media/audio/audio_system_test_util.cc
[modify] https://crrev.com/90ebce39c2a621d47c5e3c8d84bc140d01d2f67c/media/audio/audio_system_test_util.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a14d2f8335c7c0d888057cb24bbcefcbcf068081

commit a14d2f8335c7c0d888057cb24bbcefcbcf068081
Author: Chandan Padhi <c.padhi@samsung.com>
Date: Wed Dec 20 16:53:03 2017

Update MediaStreamDevice's |matched_output_device_id| to use base::Optional

|matched_output_device_id| as an empty string doesn't necessarily mean the
absence of an output device. It can also represent a default output device.
Changing |matched_output_device_id| to base::Optional will allow us to use
base::nullopt to indicate the absence of an output device.

Bug: 793255
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: If405534df9c8b558a56a1a862f6aa72ce34df5dc
Reviewed-on: https://chromium-review.googlesource.com/833828
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Chandan Padhi <c.padhi@samsung.com>
Cr-Commit-Position: refs/heads/master@{#525350}
[modify] https://crrev.com/a14d2f8335c7c0d888057cb24bbcefcbcf068081/chrome/browser/extensions/api/webrtc_audio_private/webrtc_audio_private_api.cc
[modify] https://crrev.com/a14d2f8335c7c0d888057cb24bbcefcbcf068081/chrome/browser/extensions/api/webrtc_audio_private/webrtc_audio_private_api.h
[modify] https://crrev.com/a14d2f8335c7c0d888057cb24bbcefcbcf068081/content/browser/renderer_host/media/audio_input_device_manager.cc
[modify] https://crrev.com/a14d2f8335c7c0d888057cb24bbcefcbcf068081/content/browser/renderer_host/media/audio_input_device_manager.h
[modify] https://crrev.com/a14d2f8335c7c0d888057cb24bbcefcbcf068081/content/browser/renderer_host/media/audio_output_authorization_handler.cc
[modify] https://crrev.com/a14d2f8335c7c0d888057cb24bbcefcbcf068081/content/public/common/media_stream_request.h
[modify] https://crrev.com/a14d2f8335c7c0d888057cb24bbcefcbcf068081/content/renderer/media/user_media_client_impl_unittest.cc
[modify] https://crrev.com/a14d2f8335c7c0d888057cb24bbcefcbcf068081/content/renderer/media/user_media_processor.cc
[modify] https://crrev.com/a14d2f8335c7c0d888057cb24bbcefcbcf068081/content/renderer/media/webrtc_audio_device_impl.cc
[modify] https://crrev.com/a14d2f8335c7c0d888057cb24bbcefcbcf068081/media/audio/audio_system.h
[modify] https://crrev.com/a14d2f8335c7c0d888057cb24bbcefcbcf068081/media/audio/audio_system_helper.cc
[modify] https://crrev.com/a14d2f8335c7c0d888057cb24bbcefcbcf068081/media/audio/audio_system_test_util.cc
[modify] https://crrev.com/a14d2f8335c7c0d888057cb24bbcefcbcf068081/media/audio/audio_system_test_util.h