Looks like this Chrome change has broken Cast Streaming Mac: https://chromium.googlesource.com/chromium/src/+/3956fa1cac34dd5682c271d77463accdd7191102

The change enabled the detection of hardware H264 encoders on Mac, and the OFFER/ANSWER messages in the logs confirm the Chrome Sender *is* trying to use H264.

For the short term, we'll commit a patch to revert to the old behavior (i.e., all desktop Macs will use the software VP8 encoder).

For the long term, we should understand why things are broken, as there are [the obvious] reasons to want to use the built-in hardware encoder.  Apple's VideoToolbox library may be broken/unreliable, or broken for the arbitrary resolution changes used by tab mirroring and/or Zero Config resolution changes.  Or, it could be that the Chromecast receiver (and others?) is broken for H264: again, maybe because of the arbitrary resolutions used by tab mirroring, or it is broken when changing resolutions for Zero Config.

I have a guess that it might be coming from visible_rect vs. coded_size differences. 

https://code.google.com/p/chromium/codesearch#chromium/src/media/base/mac/video_frame_mac.cc&l=65

Verified that the revert fixed this problem.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5f6bf2ab1d3619432d6d0a5021008d500b7e1c37

commit 5f6bf2ab1d3619432d6d0a5021008d500b7e1c37
Author: miu <miu@chromium.org>
Date: Mon Mar 21 23:20:04 2016

Fix Cast Streaming regression caused by enabling H264 HW encoder on Mac.

A recent commit (3956fa1cac34dd5682c271d77463accdd7191102) turned on
detection and use of the H264 hardware encoder for Mac desktop.  This
caused breakage in the form of a "black screen."  This change reverts to
the prior behavior of always using the software VP8 encoder on Macs.

BUG= 596674 

Review URL: https://codereview.chromium.org/1818203002

Cr-Commit-Position: refs/heads/master@{#382432}

[modify] https://crrev.com/5f6bf2ab1d3619432d6d0a5021008d500b7e1c37/chrome/renderer/media/cast_rtp_stream.cc
[modify] https://crrev.com/5f6bf2ab1d3619432d6d0a5021008d500b7e1c37/chrome/test/data/extensions/api_test/cast_streaming/end_to_end_sender.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/57ad17882fe82d222ffc20d8d03140c1afa366a8

commit 57ad17882fe82d222ffc20d8d03140c1afa366a8
Author: emircan <emircan@chromium.org>
Date: Tue Mar 29 02:58:20 2016

Allow padding in WrapVideoFrameInCVPixelBuffer

This CL allows visible_rect and coded_size differences in
WrapVideoFrameInCVPixelBuffer.

BUG= 596674 

Review URL: https://codereview.chromium.org/1822103003

Cr-Commit-Position: refs/heads/master@{#383662}

[modify] https://crrev.com/57ad17882fe82d222ffc20d8d03140c1afa366a8/media/base/mac/video_frame_mac.cc
[modify] https://crrev.com/57ad17882fe82d222ffc20d8d03140c1afa366a8/media/base/mac/video_frame_mac_unittests.cc

I think this has been fixed.