Tried testing the issue on mac 10.13.6 using chrome reported version #70.0.3538.77 and latest canary #72.0.3602.0.
Attached a screen cast for reference.

Following are the steps followed to reproduce the issue.
------------
1. Typed in console:
HTMLMediaElement.src = somethingTransparent.webm;
videoElement.srcObject = HTMLMediaElement.captureStream()
2. Got an Uncaught ReferenceError: somethingTransparent is not defined
    at <anonymous>:1:24.

marc@ - Could you please check the attached screen cast and please let us know if anything is missed from our end. Also if possible please provide a screenshot/screen cast for better understanding of the issue.

Thanks...!!

Deleted: 902205.mp4 442 KB

	902205.mp4 442 KB View Download

Sorry, my example wasn't very good. Here is a streamcast of the issue. and updated code:

Go here: https://simpl.info/videoalpha/
Type: video2.srcObject = video1.captureStream(); into the console.

Deleted: captureStream.mov 6.2 MB

	captureStream.mov 6.2 MB View Download

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Able to reproduce the issue on chrome version# 70.0.3538.77 also seen on 72.0.3608.0 and the issue is not seen on M-60 with the sample URL and steps provided in comment# 3 using Mac 10.14.0. We will provide bisect information and other OS behaviour soon. Hence adding Needs-Bisect label and marking it as Untriaged.

Thanks.

Able to reproduce issue on reported chrome version 70.0.3538.77 & on latest chrome 72.0.3608.0 using Mac 10.14.0, Windows 10 and Ubuntu 14.04. Hence, providing bisect information below.

Bisect Info:
================
Good build: 60.0.3169.0
Bad build:  62.0.3170.0
CHANGELOG URL:
https://chromium.googlesource.com/chromium/src/+log/5e40f323bc67b22cd168b14b019bd5dc1b48c973..c0de80b0bfdf4e18bf35433ce963fcbcda0df9ef

suspect: https://chromium.googlesource.com/chromium/src/+/c0de80b0bfdf4e18bf35433ce963fcbcda0df9ef

Reviewed-on: https://chromium-review.googlesource.com/544899

@Miguel Casas-Sanchez: Please confirm the issue and help in re-assigning if it is not related to your change.

Thanks!

 Issue 905249  has been merged into this issue.

Bisect Info:
================
Good build: 70.0.3524.0
Bad build: 70.0.3525.0

CHANGELOG URL:
You are probably looking for a change made after 583622 (known good), but no later than 583623 (first known bad).
https://chromium.googlesource.com/chromium/src/+log/8d58277248cb981741eb4a48d2938719880bee4e..1e53b4ec05f2485450a177c372e38b6215341a1c

Suspect: https://chromium.googlesource.com/chromium/src/+/1e53b4ec05f2485450a177c372e38b6215341a1c

Reviewed-on: https://chromium-review.googlesource.com/1174949

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fb63a5791874c6398f709edd77b35dfd1b8efad6

commit fb63a5791874c6398f709edd77b35dfd1b8efad6
Author: Miguel Casas <mcasas@chromium.org>
Date: Tue Dec 04 23:50:41 2018

HtmlVideoElementCapturerSource: Add opacity/alpha source support

HtmlVideoElementCapturerSource, the impl of <video>.captureStream(),
ignores the opacity/transparency of the source on ToT. causing the
bug below.  This CL adds support for I420A in the said Source, and
for that also adds an IsOpaque() method to WebMediaPlayer and to
its WMPImpl.

Bug:  902205 
Change-Id: Ic14119b7df54ded4bee9544df9de90a3628db6df
Reviewed-on: https://chromium-review.googlesource.com/c/1359513
Reviewed-by: Emircan Uysaler <emircan@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613764}
[modify] https://crrev.com/fb63a5791874c6398f709edd77b35dfd1b8efad6/content/renderer/media_capture_from_element/html_video_element_capturer_source.cc
[modify] https://crrev.com/fb63a5791874c6398f709edd77b35dfd1b8efad6/content/renderer/media_capture_from_element/html_video_element_capturer_source.h
[modify] https://crrev.com/fb63a5791874c6398f709edd77b35dfd1b8efad6/content/renderer/media_capture_from_element/html_video_element_capturer_source_unittest.cc
[modify] https://crrev.com/fb63a5791874c6398f709edd77b35dfd1b8efad6/media/blink/webmediaplayer_impl.cc
[modify] https://crrev.com/fb63a5791874c6398f709edd77b35dfd1b8efad6/media/blink/webmediaplayer_impl.h
[modify] https://crrev.com/fb63a5791874c6398f709edd77b35dfd1b8efad6/third_party/blink/public/platform/web_media_player.h

Someone plz verify when #11 gets to Canary

I've just tested this now in Canary Version 73.0.3632.0 and its working perfectly. Thanks heaps. 

 Issue 905249  has been merged into this issue.