Mark a layout test...

Mark a layout test external/wpt/offscreen-canvas/the-offscreen-canvas/offscreencanvas.convert.to.blob.html as flaky

Bug: 813462
Change-Id: I1218269ed75ff50b991505f6f67f770c29e2d593
Reviewed-on: https://chromium-review.googlesource.com/923743
Commit-Queue: Takeshi Yoshino <tyoshino@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537597}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3207,6 +3207,7 @@ crbug.com/812417 [ Win7 ] http/tests/devtools/tracing/timeline-style/timeline-re
 
 # Sheriff failures 2018-02-19
 crbug.com/771643 [ Mac10.11 ] virtual/gpu-rasterization/images/color-profile-image-filter-all.html [ Failure ]
+crbug.com/813462 external/wpt/offscreen-canvas/the-offscreen-canvas/offscreencanvas.convert.to.blob.html [ Pass Timeout ]
 
 # These pass on bots with proprietary codecs (most CQ bots) while failing on bots without.
 crbug.com/807110 external/wpt/html/semantics/embedded-content/media-elements/mime-types/canPlayType.html [ Failure Pass ]