When navigating to another site while mirroring to Hendrix, WebContentsTracker::RenderFrameDeleted() and WebContentsTracker::WebContentsDestroyed() are called, which indicates no RenderWidgetHost. And therefore session ends.

miu: Do you know possible cause for this? I tried mirroring to ChromeCast and navigated away, these two methods are not called, and instead, WebContentsTracker::RenderFrameHostChanged() is called.

From some code sleuthing, I'm guessing we're not incrementing the capturer count on the WebContents for audio only capture.  That prevents prerendering, which is implemented by swapping WebContents (destroying the current one and swapping in a pre-rendered one on navigation).

It looks like the capturer count is incremented only in the video capture implementation, I strongly suspect my hunch is correct.

Possible solutions:
- Create a fake video capturer just to increment the count
- Plumb in a call to increment the capture count for audio capture as well.

The latter feels better, but I don't see a way to easily get a pointer to the WebContents being captured in the audio capture code I browsed.

WebContentsAudioInputStream has the access to WebContents through |WebContentsTracker|, which can increment the count. I will try to see if it works.

It appears the capture code appears to be re-purposed for tab muting.  I wonder if we need to also block prerendering for muted tabs.  miu@ should definitely be looped in.
 

xjz asked me about this. I had two suspicions:

1. The behavior was coincidential (i.e., GetCapturerCount() is *not* a ref-count that must go to zero; and so it could be that sometimes a WebContents was being destroyed due to navigation logic). I chatted with creis@, and confirmed this is not the case.

2. Prerendering: Looks like this *is* the culprit, since prerendering checks the capture count and will abort if >0.

So, talked with xjz about a fix that will make sure the WebContentsAudioInputStream is incrementing the capture count (currently, only the video capture "device" does so).

Patch out for review: https://codereview.chromium.org/2460033003/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c227f1bb6dc4e286df809a97abaef9bdef63c19e

commit c227f1bb6dc4e286df809a97abaef9bdef63c19e
Author: xjz <xjz@chromium.org>
Date: Sat Oct 29 01:09:30 2016

Increment/Decrement capturer count when starts/stops audio capturing.

BUG= 656135 

Review-Url: https://codereview.chromium.org/2460033003
Cr-Commit-Position: refs/heads/master@{#428576}

[modify] https://crrev.com/c227f1bb6dc4e286df809a97abaef9bdef63c19e/content/browser/media/capture/web_contents_audio_input_stream.cc

We should probably merge into M55 as well.

Is this applicable to all OSs or any specific OS?

All desktop OSes. (I updated OS labels.)

Your change meets the bar and is auto-approved for M55 (branch: 2883)

**** Bulk edit -  please ignore if not applicable ****

Please merge your change to M55 branch 2883 today before 5:00 PM PT or latest by tomorrow, Tuesday (11/01/16) 4:00 PM PT so we can take it for this week Beta release. 

I am still not allowed to do merging.
Fatal push error. Make sure your .netrc credentials and git user.email are correct and you have push access to the repo.

miu: Can you again help me do this?
git drover --branch 2883 --cherry-pick c227f1bb6dc4e286df809a97abaef9bdef63c19e

Thanks!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7862fa6644ca8da60af147ddbb6a84008bdf276f

commit 7862fa6644ca8da60af147ddbb6a84008bdf276f
Author: Yuri Wiitala <miu@chromium.org>
Date: Mon Oct 31 19:49:30 2016

Increment/Decrement capturer count when starts/stops audio capturing.

BUG= 656135 

Review-Url: https://codereview.chromium.org/2460033003
Cr-Commit-Position: refs/heads/master@{#428576}
(cherry picked from commit c227f1bb6dc4e286df809a97abaef9bdef63c19e)

Review URL: https://codereview.chromium.org/2465013002 .

Cr-Commit-Position: refs/branch-heads/2883@{#392}
Cr-Branched-From: 614d31daee2f61b0180df403a8ad43f20b9f6dd7-refs/heads/master@{#423768}

[modify] https://crrev.com/7862fa6644ca8da60af147ddbb6a84008bdf276f/content/browser/media/capture/web_contents_audio_input_stream.cc

Verified with Chrome: 56.0.2922.1