Make tab audible and muted states and cause available for an extension API

content/browser/media/capture/web_contents_audio_input_stream.cc

 // Copyright (c) 2013 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.
 
 #include "content/browser/media/capture/web_contents_audio_input_stream.h"
 
 #include <string>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
@@ skipping 233 matching lines @@
       base::Bind(&AudioMirroringManager::StopMirroring,
                  base::Unretained(mirroring_manager_),
                  make_scoped_refptr(this)));
 }
 
 void WebContentsAudioInputStream::Impl::UnmuteWebContentsAudio() {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
   WebContents* const contents = tracker_->web_contents();
   if (contents)
-    contents->SetAudioMuted(false);
+    contents->SetAudioMuted(false, "user");

[miu, 2014/12/05 23:46:48]

You won't be able to reference that string constant in tab_utils.cc from here. 
However, IMO, this should probably be a different string anyway.  How about (at
top of this file):

  static const char kForceUnmutedCause[] = "auto-forced for capture";

 }
 
 void WebContentsAudioInputStream::Impl::QueryForMatches(
     const std::set<SourceFrameRef>& candidates,
     const MatchesCallback& results_callback) {
   BrowserThread::PostTask(
       BrowserThread::UI,
       FROM_HERE,
       base::Bind(&Impl::QueryForMatchesOnUIThread,
                  this,
@@ skipping 123 matching lines @@
 
 bool WebContentsAudioInputStream::GetAutomaticGainControl() {
   return impl_->mixer_stream()->GetAutomaticGainControl();
 }
 
 bool WebContentsAudioInputStream::IsMuted() {
   return false;
 }
 
 }  // namespace content