Found the root cause, and uploaded a fix for code review: https://chromium-review.googlesource.com/c/chromium/src/+/1068614

The root-cause was in viz::GLRendererCopier, where the scaler was not being configured to Y-flip its output when the source texture was not Y-flipped. The reason this is required is because the next stage of the copier pipeline assumes all of its textures to already be Y-flipped.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c938d815ed9a6804fd7dd38499762ac5c46d9388

commit c938d815ed9a6804fd7dd38499762ac5c46d9388
Author: Yuri Wiitala <miu@chromium.org>
Date: Tue May 22 17:08:00 2018

Fix upside-down browser window capture on Windows.

The root-cause was in viz::GLRendererCopier, where the scaler was not
being configured to Y-flip its output when the source texture was not
Y-flipped. The reason this is required is because the next stage of the
copier pipeline assumes all of its textures to already be Y-flipped.

TEST=On all of Win/CrOS/Linux, run src/chrome/common/extensions/docs/examples/api/desktopCapture example, choosing a browser window.

Bug:  845349 
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I18b6f1e0c012bd9e87814881f7bfde85305e250b
Reviewed-on: https://chromium-review.googlesource.com/1068614
Commit-Queue: Xiangjun Zhang <xjz@chromium.org>
Reviewed-by: Xiangjun Zhang <xjz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560664}
[modify] https://crrev.com/c938d815ed9a6804fd7dd38499762ac5c46d9388/components/viz/service/display/gl_renderer_copier.cc
[modify] https://crrev.com/c938d815ed9a6804fd7dd38499762ac5c46d9388/components/viz/service/display/gl_renderer_copier.h

I can't tell if this fix is in CrOS dev version 68.0.3437.0 or not. But when I cast the desktop, it is upside down and reversed (shown from the backside of the image). 

Deleted: desktop_flipped.jpg 2.8 MB

	desktop_flipped.jpg 2.8 MB View Download

That version does not have the fix. 3437 branched 2 days prior to the fix landing.

miu@ -- Could you please provide us the steps to verify this issue and sample extension to verify the issue. This would help us in verifying the fix at TE's end and add TE-Verified labels.

Thanks in advance!

I verified this was fixed by:

- join a hangouts meet meeting and screenshare a different (youtube) window on the system
- join the same hangouts meet meeting from somewhere else and observe the viewed screenshare is right-side-up

 Issue 847074  has been merged into this issue.

Issue 847885 has been merged into this issue.

I verified this was fixed in 68.0.3440.15