Marked OffscreenCanvas-2d-imageSmoothing.html as flaky

That is, marked
virtual/gpu/fast/canvas/OffscreenCanvas-2d-imageSmoothing.html
as [ Pass Failure ] for [ Linux ].

TBR=junov@chromium.org

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 789533
Change-Id: I633fa4e9d24e796969a2fb950ce78da0b8170984
Reviewed-on: https://chromium-review.googlesource.com/796476Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520096}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4066,3 +4066,4 @@ crbug.com/785179 [ Mac10.11 ] http/tests/devtools/console/console-viewport-stick
 # Sheriff failures 2017-11-29
 crbug.com/789478 [ Mac ] virtual/prefer_compositing_to_lcd_text/scrollbars/custom-scrollbar-adjust-on-inactive-pseudo.html [ Pass Failure ]
 crbug.com/789485 [ Linux ] virtual/mouseevent_fractional/fast/events/touch/gesture/gesture-tap-mouse-events-between-frames.html [ Pass Failure ]
+crbug.com/789533 [ Linux ] virtual/gpu/fast/canvas/OffscreenCanvas-2d-imageSmoothing.html [ Pass Failure ]