[Sheriff] Disable offscreencanvas-webgl-printing.html

Disabled on Mac due to flakiness

TBR=aaronhk@chromium.org

Bug: 920907
Change-Id: Ib3b4df696dbb7b7a1a2a4f214b2c9bd67d12d5b0
Reviewed-on: https://chromium-review.googlesource.com/c/1407070Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622029}

third_party/blink/web_tests/TestExpectations

@@ -307,6 +307,8 @@ crbug.com/736052 virtual/stable/compositing/overflow/composited-scroll-with-frac
 crbug.com/680635 external/wpt/offscreen-canvas/text/2d.text.font.parse.invalid.html [ Skip ]
 crbug.com/680635 external/wpt/offscreen-canvas/text/2d.text.font.parse.invalid.worker.html [ Skip ]
 
+crbug.com/920907 [ Mac ] virtual/threaded/printing/offscreencanvas-webgl-printing.html [ Pass Failure ]
+
 # With removing fling handling logic from renderer Layouttests that are using event sender to send fling events are failing.
 crbug.com/857490 fast/events/touch/gesture/pad-gesture-cancel.html [ Skip ]
 crbug.com/857490 fast/events/touch/gesture/pad-gesture-fling.html [ Skip ]