[Sheriff] Mark OffscreenCanvas-placeholder-createImageBitmap.html as flaky

Two versions of this test have been flaking on Mac and causing CQ false
rejections.

Bug: 1019079
Change-Id: Ib6a69539a3e695e59d4c450bba10e91a94b97758
No-Try: True
TBR: juanmihd@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1943243Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720230}

third_party/blink/web_tests/TestExpectations

@@ -5853,6 +5853,8 @@ crbug.com/1015187 [ Linux Mac ] external/wpt/html/cross-origin-embedder-policy/n
 
 # Sheriff 2019-11-29
 crbug.com/1027436 [ Mac ] inspector-protocol/heap-profiler/heap-snapshot-with-detached-dom-tree.js [ Pass Timeout ]
+crbug.com/1019079 [ Mac ] virtual/gpu/fast/canvas/OffscreenCanvas-placeholder-createImageBitmap.html [ Pass Failure ]
+crbug.com/1019079 [ Mac ] fast/canvas/OffscreenCanvas-placeholder-createImageBitmap.html [ Pass Failure ]
 
 # Temporary suppression to allow Skia change to opaqueness tracking
 crbug.com/1028116 virtual/gpu-rasterization-disable-yuv/images/yuv-decode-eligible/color-profile-border-radius.html [ Pass Failure ]