Update: for more investigation, we've found that only the nonexistence of speakers will impact the system audio capture. The existence of microphones has nothing to do with system audio capture.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/14c1ec38f971f2dd61ec038284ea65d9949b5e86

commit 14c1ec38f971f2dd61ec038284ea65d9949b5e86
Author: Qiang Chen <qiangchen@google.com>
Date: Tue Jun 27 17:47:09 2017

Bug Fix: System audio capture abnormal when microphones missing

When querying audio parameters for a given device id, we check the existence of microphones, which is correct if we want to capture microphone stream.
But it is not a correct check, if we need to capture the system audio, we should instead guarantee the existence of a speaker device.

Bug: 736016
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: I480046f1ed69f7b275fc368d8dd6fd73b7bac0ff
Reviewed-on: https://chromium-review.googlesource.com/549077
Reviewed-by: Tommi Gunn <tommi@chromium.org>
Commit-Queue: Tommi Gunn <tommi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#482672}
[modify] https://crrev.com/14c1ec38f971f2dd61ec038284ea65d9949b5e86/media/audio/audio_system_impl.cc

I'm not sure the change is a good design decision from AudioSystem perspective, and I'm not happy about the fact that it landed before we reached an agreement on how to approach it.
For the history of the discussion see  Issue 733356 .

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/67c685048f7e02e39c72608f3f7d3d0bb6c20ac2

commit 67c685048f7e02e39c72608f3f7d3d0bb6c20ac2
Author: Yuri Wiitala <miu@chromium.org>
Date: Tue Jun 27 20:25:43 2017

Revert "Bug Fix: System audio capture abnormal when microphones missing"

This reverts commit 14c1ec38f971f2dd61ec038284ea65d9949b5e86.

Reason for revert: Code review not complete. Outstanding concerns about the approach.

Original change's description:
> Bug Fix: System audio capture abnormal when microphones missing
> 
> When querying audio parameters for a given device id, we check the existence of microphones, which is correct if we want to capture microphone stream.
> But it is not a correct check, if we need to capture the system audio, we should instead guarantee the existence of a speaker device.
> 
> Bug: 736016
> 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: I480046f1ed69f7b275fc368d8dd6fd73b7bac0ff
> Reviewed-on: https://chromium-review.googlesource.com/549077
> Reviewed-by: Tommi Gunn <tommi@chromium.org>
> Commit-Queue: Tommi Gunn <tommi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#482672}

TBR=tommi@chromium.org,miu@chromium.org,qiangchen@chromium.org

Change-Id: If6ca412a19112a6f07cc58640ebfcd96928637c0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 736016
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
Reviewed-on: https://chromium-review.googlesource.com/550675
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#482730}
[modify] https://crrev.com/67c685048f7e02e39c72608f3f7d3d0bb6c20ac2/media/audio/audio_system_impl.cc