The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a282a4569fe0786fe2bec8c9d0bfbcadff276330

commit a282a4569fe0786fe2bec8c9d0bfbcadff276330
Author: qiangchen <qiangchen@chromium.org>
Date: Fri Jul 29 17:22:01 2016

MuteSource Audio During Full Screen Cast

Previously, when casting a chrome tab, the source audio is
automatically muted; however, casting the full screen, the
system audio is still audible.

This CL makes the behavior consistent.

P.S. But the technique is different for muting a tab and muting system audio.
1. For muting tab, we divert the audio stream from speaker
to WebContentsAudioInputStream.
2. For muting system audio, we call the OS API to do so,
which is just as if the user click the "mute" button on
the task bar.

Visible difference is that during capturing, the user is
not able to unmute tab audio on the capture side, but
nothing can prevent the end user to unmute system audio.

BUG= 629561 

NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2144333002
Cr-Commit-Position: refs/heads/master@{#408672}

[modify] https://crrev.com/a282a4569fe0786fe2bec8c9d0bfbcadff276330/chrome/browser/media/desktop_capture_access_handler.cc
[modify] https://crrev.com/a282a4569fe0786fe2bec8c9d0bfbcadff276330/content/browser/renderer_host/media/audio_input_renderer_host.cc
[modify] https://crrev.com/a282a4569fe0786fe2bec8c9d0bfbcadff276330/media/audio/audio_device_description.cc
[modify] https://crrev.com/a282a4569fe0786fe2bec8c9d0bfbcadff276330/media/audio/audio_device_description.h
[modify] https://crrev.com/a282a4569fe0786fe2bec8c9d0bfbcadff276330/media/audio/cras/cras_input.cc
[modify] https://crrev.com/a282a4569fe0786fe2bec8c9d0bfbcadff276330/media/audio/cras/cras_input.h
[modify] https://crrev.com/a282a4569fe0786fe2bec8c9d0bfbcadff276330/media/audio/win/audio_low_latency_input_win.cc
[modify] https://crrev.com/a282a4569fe0786fe2bec8c9d0bfbcadff276330/media/audio/win/audio_low_latency_input_win.h
[modify] https://crrev.com/a282a4569fe0786fe2bec8c9d0bfbcadff276330/media/audio/win/core_audio_util_win.cc