[sheriff] Mark canvas test as flaky

virtual/gpu/fast/canvas/canvas-composite-shadow.html is flaky.

Mark as such.

Tbr: sugoi@chromium.org
Bug: 1150518
Change-Id: I0ee14f9d8f4c38fe4c61cc0cd1e697f4e8249f0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2547391Reviewed-by: Nina Satragno <nsatragno@chromium.org>
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828891}

third_party/blink/web_tests/TestExpectations

@@ -6002,6 +6002,7 @@ crbug.com/987138 [ Win ] media/controls/doubletap-to-jump-forwards.html [ Pass T
 crbug.com/1146221 [ Linux ] http/tests/devtools/sources/debugger-ui/debugger-inline-values.js [ Pass Timeout ]
 
 # Sheriff 2020-11-18
+crbug.com/1150518 [ Win ] virtual/gpu/fast/canvas/canvas-composite-shadow.html [ Pass Timeout ]
 crbug.com/915903 http/tests/security/inactive-document-with-empty-security-origin.html [ Pass Timeout ]
 crbug.com/1105957 [ Linux Debug ] fast/dom/cssTarget-crash.html [ Pass Timeout ]
 crbug.com/1150475 fast/dom/open-and-close-by-DOM.html [ Pass Failure ]
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