canvas-composited-stroke-alpha is flaky for Mac, Linux and Windows

Fixing the expectation so it covers all platforms where it is flaky.

Bug: 984467
Change-Id: I5daf2f5f02e6140163d659a75b74a39d24f7c3a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2228137
Auto-Submit: Juanmi Huertas <juanmihd@chromium.org>
Reviewed-by: Yi Xu <yiyix@chromium.org>
Commit-Queue: Juanmi Huertas <juanmihd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774713}

third_party/blink/web_tests/TestExpectations

@@ -5839,7 +5839,7 @@ crbug.com/1068681 fast/peerconnection/RTCPeerConnection-insertable-streams.html
 
 # Sheriff 2020-05-04
 crbug.com/952717 [ Debug ] http/tests/xmlhttprequest/redirect-cross-origin-post.html [ Pass Failure ]
-crbug.com/984467 [ Mac Debug ] virtual/gpu/fast/canvas/canvas-composite-stroke-alpha.html [ Pass Timeout ]
+crbug.com/984467 virtual/gpu/fast/canvas/canvas-composite-stroke-alpha.html [ Pass Timeout ]
 
 # Sheriff 2020-05-06
 crbug.com/1078863 [ Mac ] fast/speech/scripted/speechrecognition-restart-onend.html [ Pass Timeout ]