Ignore snav-tiny-table-traversal.html failures for Mac

TBR=mkwst@chromium.org

Bug: 970334
Change-Id: I10c31612c6623d50382f485fd26845a2f4c8faa1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1643498Reviewed-by: Dan H <harringtond@chromium.org>
Commit-Queue: Dan H <harringtond@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666058}

third_party/blink/web_tests/TestExpectations

@@ -5865,5 +5865,6 @@ crbug.com/905971 [ Win ] virtual/blink-cors/external/wpt/service-workers/service
 
 # Sheriff 2019-06-04
 crbug.com/970135 [ Mac ] virtual/focusless-spat-nav/fast/spatial-navigation/focusless/snav-focusless-interested-element-indicated.html [ Failure ]
+crbug.com/970334 [ Mac ] fast/spatial-navigation/snav-tiny-table-traversal.html [ Failure ]
 crbug.com/970142 http/tests/security/mixedContent/insecure-css-resources.html [ Failure ]
 crbug.com/970142 virtual/blink-cors/http/tests/security/mixedContent/insecure-css-resources.html [ Failure ]