Mark offscreencanvas.convert.to.blob.html flaky on leak bot

TBR=thestig@chromium.org, xlai@chromium.org
NOTRY=true

Bug: 836278
Change-Id: I4fba063d1782c9d623a2c57ba8bfaf1fa52abadd
Reviewed-on: https://chromium-review.googlesource.com/1026088Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553156}

third_party/WebKit/LayoutTests/LeakExpectations

@@ -72,6 +72,8 @@ crbug.com/821547 [ Linux ] virtual/threaded/animations/interpolation/filter-inte
 # Sheriff 2018-04-23
 # -----------------------------------------------------------------
 crbug.com/835802 [ Linux ] fast/loader/scroll-position-restored-on-back-at-load-event.html [ Pass Leak ]
+crbug.com/836278 [ Linux ] external/wpt/offscreen-canvas/convert-to-blob/offscreencanvas.convert.to.blob.html [ Pass Leak ]
+crbug.com/836278 [ Linux ] virtual/threaded/external/wpt/offscreen-canvas/convert-to-blob/offscreencanvas.convert.to.blob.html [ Pass Leak ]
 
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 # WARNING: Memory leaks must be fixed asap. Sheriff is expected to revert #