Yes I noticed that too on last Friday.
Going to look into it.(Having problem to build chromium on my Linux box though...)

It turns out to be an existing problem in chrome.desktopCapture.cancelChooseDesktopMedia, or in Chrome Extensions, with debug building.

Those error logs comes from 'cancelDialog()' case, which will call cancelChooseDesktopMedia() eventually at https://cs.chromium.org/chromium/src/chrome/test/data/extensions/api_test/desktop_capture/test.js?sq=package:chromium&dr&l=122.
Within cancelChooseDesktopMedia(), it will call "RespondNow(NoArguments())" as return, in which causes ShemaUtils.validate error at https://cs.chromium.org/chromium/src/extensions/renderer/resources/schema_utils.js?l=36.

I'm contacting Extension guys to see what can be done.

Thanks to rdcronin@! He helped to figure out the problem is actually in DesktopCaptureChooseDesktopMediaFunctionBase::Cancel(), https://cs.chromium.org/chromium/src/chrome/browser/extensions/api/desktop_capture/desktop_capture_base.cc?l=73.
Since the shema has two parameters, https://cs.chromium.org/chromium/src/chrome/common/extensions/api/desktop_capture.json?l=38, so whenever we set the result, those two parameters must be passed in, otherwise shemaUtils.validate() will throw an error.
I guess this problem has existed for about one year since the option for audio capture was added. 
A cl is uploaded, waiting for passing try bots.

Great! Do we understand why the test passes in spite of the cancel case not
actually working as intended?

I guess the js exception won't block the test and we eventually give "chrome.test.succeed();" in cancelDialog() case, https://cs.chromium.org/chromium/src/chrome/test/data/extensions/api_test/desktop_capture/test.js?sq=package:chromium&dr&l=123.

Shouldn't the cancelChooseDesktopMedia() call have trigger a JavaScript
exception, preventing us from reaching the chrome.test.succeed() line,
though?  Or are schema errors somehow not reported back to JS?

It looks like that I can't catch the thrown error in test.js.
No idea about the Extensions implementation at all. Any insight?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e334368d858f03a8fd653f72eebf37675c16d585

commit e334368d858f03a8fd653f72eebf37675c16d585
Author: Weiyong Yao <braveyao@chromium.org>
Date: Thu Nov 16 01:59:32 2017

[desktopCapture] set expected results when a dialog is cancelled.

The desktop capture has the shema[1] that has two parameters, a streamId
and an option object which is added laterly. But when we cancle the picker
dialog, the option object isn't added into the result in Cancel()[2].
So schemaUtils.validate() will throw an error due to the violation.

This CL addes an option object into the result list to meet the requirement.

[1]: https://cs.chromium.org/chromium/src/chrome/common/extensions/api/desktop_capture.json?l=38
[2]: https://cs.chromium.org/chromium/src/chrome/browser/extensions/api/desktop_capture/desktop_capture_base.cc?l=73

Bug:  784508 
Change-Id: Ie17743dacff5f0171af52445df673b70e89a62cd
Reviewed-on: https://chromium-review.googlesource.com/773115
Reviewed-by: Zijie He <zijiehe@chromium.org>
Commit-Queue: Weiyong Yao <braveyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516952}
[modify] https://crrev.com/e334368d858f03a8fd653f72eebf37675c16d585/chrome/browser/extensions/api/desktop_capture/desktop_capture_base.cc