This is happening on linux bots:
gpu: none
os: Ubuntu-14.04
cpu: x86-64
pool: Chrome

The test keeps timing out:
08:13:00.970 12188 killed pid 12769
08:13:00.973 12188 worker/2 virtual/gpu/fast/canvas/canvas-composite-video-shadow.html output stderr lines:
08:13:00.973 12188   
08:13:00.973 12188   DevTools listening on ws://127.0.0.1:44415/devtools/browser/168cf978-8ee7-4901-b220-da15f5bb2289
08:13:00.976 1692 [11/12] virtual/gpu/fast/canvas/canvas-composite-video-shadow.html failed unexpectedly (test timed out)
08:13:00.974 12188 worker/2 killing primary driver
08:13:00.975 12188 worker/2 killing secondary driver
08:13:00.975 12188 worker/2 virtual/gpu/fast/canvas/canvas-composite-video-shadow.html failed:
08:13:00.975 12188 worker/2  test timed out
08:13:00.975 12188 worker/2 killing secondary driver
08:13:00.975 12188 worker/2 exiting
08:13:00.975 12188 worker/2 cleaning up
08:13:00.976 12188 worker/2 killing primary driver
08:13:00.976 12188 worker/2 killing secondary driver

This test is disable on windows already:

https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/TestExpectations?l=4119
https://bugs.chromium.org/p/chromium/issues/detail?id=757165

Maybe it should be disable on linux too?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/193e138de752d19eea42e2833db5961e19551a41

commit 193e138de752d19eea42e2833db5961e19551a41
Author: Roger Tawa <rogerta@chromium.org>
Date: Mon Jul 30 17:53:37 2018

"virtual/gpu/fast/canvas/canvas-composite-video-shadow.html" is flaky.
Disabling test on linux.

BUG= 868956 
TBR=qyearsley@chromium.org,jeffcarp@chromium.org,dpranke@chromium.org,tkent@chromium.org
NOTRY=true

Change-Id: I7b1639a1dc94ff139b696633209fb72e75f92338
Reviewed-on: https://chromium-review.googlesource.com/1155103
Commit-Queue: Roger Tawa <rogerta@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579084}
[modify] https://crrev.com/193e138de752d19eea42e2833db5961e19551a41/third_party/WebKit/LayoutTests/TestExpectations

Detected 3 new flakes for test/step "virtual/gpu/fast/canvas/canvas-composite-video-shadow.html". To see the actual flakes, please visit https://chromium-try-flakes.appspot.com/all_flake_occurrences?key=ahVzfmNocm9taXVtLXRyeS1mbGFrZXNyRQsSBUZsYWtlIjp2aXJ0dWFsL2dwdS9mYXN0L2NhbnZhcy9jYW52YXMtY29tcG9zaXRlLXZpZGVvLXNoYWRvdy5odG1sDA. This message was posted automatically by the chromium-try-flakes app. Since flakiness is ongoing, the issue was moved back into Sheriff Bug Queue (unless already there).

Actually, this test seems just slow, and the SLOW annotation may make the test
pass again.

I'm going to adjust the test expectation.

TestExpectation change in CQ (soon to land)
https://chromium-review.googlesource.com/c/chromium/src/+/1195185

Let's see if the test starts passing later.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4507c19b8eb735a7ad711903f3f96c985701bac0

commit 4507c19b8eb735a7ad711903f3f96c985701bac0
Author: Yuta Kitamura <yutak@chromium.org>
Date: Wed Aug 29 07:52:03 2018

Mark canvas-composite-video-shadow.html as Slow.

This test seems consistently slow on different platforms. Let's try to
give it a longer timeout and see if it starts to pass again.

TBR=nednguyen@chromium.org

Bug:  868956 
Change-Id: I9c22bf1ae52dcfec47f9ad248c6de0c767dbbf7b
Reviewed-on: https://chromium-review.googlesource.com/1195185
Reviewed-by: Yuta Kitamura <yutak@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587034}
[modify] https://crrev.com/4507c19b8eb735a7ad711903f3f96c985701bac0/third_party/WebKit/LayoutTests/SlowTests
[modify] https://crrev.com/4507c19b8eb735a7ad711903f3f96c985701bac0/third_party/WebKit/LayoutTests/TestExpectations

Seems mostly OK for now.