Update canvas-imageSmoothingQuality.html layout test fail status

According to the flakiness dashboard, layout test virtual/gpu/fast/canvas/
canvas-imageSmoothingQuality.html is now passing on Linux and Win, but still
failing on Mac trybots:

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_tests&tests=virtual%2Fgpu%2Ffast%2Fcanvas%2Fcanvas-imageSmoothingQuality.html

This CL updates the fail tag to reflect this behavior.

Bug: 693568
Change-Id: Ibd2974c9c02706d0ad5d6b0c342e2dc1112ead34
TBR: junov@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/768948Reviewed-by: Mohammad Reza Zakerinasab <zakerinasab@chromium.org>
Commit-Queue: Mohammad Reza Zakerinasab <zakerinasab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516338}

third_party/WebKit/LayoutTests/TestExpectations

@@ -237,7 +237,7 @@ crbug.com/762054 external/wpt/css/cssom-view/overscrollBehavior-manual.html [ Sk
 # Fails consistently on WebKit Mac10.10, WebKit Mac10.11 (retina) and mac10.10_blink_rel tryserver, but not on other Mac bots.
 crbug.com/614910 [ Mac ] virtual/gpu-rasterization/images/pixel-crack-image-background-webkit-transform-scale.html [ Pass Failure ]
 
-crbug.com/693568 virtual/gpu/fast/canvas/canvas-imageSmoothingQuality.html [ Failure ]
+crbug.com/693568 [ Mac ] virtual/gpu/fast/canvas/canvas-imageSmoothingQuality.html [ Failure ]
 
 # Looks like a filuare to get a paint on time. Test could be changed to use runAfterLayoutAndPaint
 # instead of a timeout, which may fix things.