Mark media/video-transformed.html flaky on Mac

TBR=khushalsagar@chromium.org

No-Presubmit: true
No-Tree-Checks: true
No-Try: true

Bug: 813545, 813150
Change-Id: I46869c1d8bb4250fc94416c9083e0d2185d9b8a1
Reviewed-on: https://chromium-review.googlesource.com/926402
Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
Reviewed-by: Maxim Kolosovskiy <kolos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537778}

third_party/WebKit/LayoutTests/TestExpectations

@@ -1060,7 +1060,7 @@ crbug.com/492664 external/wpt/css/css-writing-modes/clip-rect-vrl-006.xht [ Fail
 crbug.com/492664 external/wpt/css/css-writing-modes/clip-rect-vrl-008.xht [ Failure ]
 crbug.com/492664 external/wpt/css/css-writing-modes/line-box-direction-vrl-009.xht [ Failure ]
 
-crbug.com/813545 [ Win7 ] media/video-transformed.html [ Pass Failure ]
+crbug.com/813545 [ Win7 Mac ] media/video-transformed.html [ Pass Failure ]
 crbug.com/637255 [ Win10 ] media/video-transformed.html [ Pass Failure ]
 
 # These tests pass but images do not match because tests are stricter than the spec.