Mark flaky Mac GPU dependent tests

These appear to be issues with the GPU used on the machine.

TBR=kbr@chromium.org

Change-Id: I5009748dad3de63acbbdcc3b7920f09a2fdfaa80
Reviewed-on: https://chromium-review.googlesource.com/996866Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548431}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3353,6 +3353,11 @@ crbug.com/736177 [ Mac10.12 Retina ] tables/mozilla_expected_failures/core/capti
 crbug.com/736177 [ Mac10.12 Retina ] tables/mozilla_expected_failures/core/captions2.html [ Failure Pass ]
 crbug.com/736177 [ Mac10.12 Retina ] virtual/prefer_compositing_to_lcd_text/compositing/overflow/theme-affects-visual-overflow.html [ Failure Pass ]
 crbug.com/736177 [ Mac10.12 Retina ] virtual/scalefactor200/fast/hidpi/static/validation-bubble-appearance-hidpi.html [ Failure Pass ]
+crbug.com/736177 [ Mac ] virtual/gpu/fast/canvas/canvas-composite-shadow.html [ Failure Pass ]
+crbug.com/736177 [ Mac ] virtual/gpu/fast/canvas/canvas-composite-video.html [ Failure Pass ]
+crbug.com/736177 [ Mac ] virtual/gpu/fast/canvas/canvas-pattern-no-repeat-with-transformations.html [ Failure Pass ]
+crbug.com/736177 [ Mac ] virtual/gpu/fast/canvas/fillrect_gradient.html [ Failure Pass ]
+crbug.com/736177 [ Mac ] virtual/gpu/fast/canvas/canvas-composite-video-shadow.html [ Failure Pass ]
 
 crbug.com/734762 inspector-protocol/timeline/page-frames.js [ Failure ]