These tests were already marked as Slow for Win Debug, but they are timing out on Mac Debug and Linux Debug as well. I am marking the test as such in https://crrev.com/c/951689. brucedawson@, I added you as TBR since you were the owner of the seemingly related  https://crbug.com/647192 . Do you mind taking a look at this as well?

 Issue 819640  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a39407cb7a14da9efe90db3f2a8f527593ea6b7a

commit a39407cb7a14da9efe90db3f2a8f527593ea6b7a
Author: jdoerrie <jdoerrie@chromium.org>
Date: Wed Mar 07 16:42:52 2018

Mark webexposed/global-interface-listing.html as Slow on all platforms

TBR=brucedawson

Bug:  819641 
Change-Id: I5a24cba7a89ef43e0c0ce762680eaf9e2ab6aa0a
Reviewed-on: https://chromium-review.googlesource.com/951689
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541468}
[modify] https://crrev.com/a39407cb7a14da9efe90db3f2a8f527593ea6b7a/third_party/WebKit/LayoutTests/SlowTests

My initial impression is that this is not worth investigating. Debug builds are always slow and that's okay. The extra slowdown may be due to some added debug checks and I'm not sure that we'd want to remove them if tagging them as slow works.

Reasonable?

Debug builds are slow, WAI. Closing as WontFix.