Disable http/tests/devtools/elements/styles-3/styles-add-blank-property.js

Disable the test on Mac and Win7.

Bug: 938884
Tbr: petewil@chromium.org
Change-Id: I107a836a9473df71ea6b70fe345d1156015fd89a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506005Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638173}

third_party/blink/web_tests/TestExpectations

@@ -6004,3 +6004,4 @@ crbug.com/938780 [ Mac ] external/wpt/animation-worklet/worklet-animation-with-s
 
 # Sheriff 2019-03-06
 crbug.com/938591 [ Linux ] fast/events/middleClickAutoscroll-nested-divs-forbidden.html [ Failure Pass ]
+crbug.com/938884 [ Win7 Mac ] http/tests/devtools/elements/styles-3/styles-add-blank-property.js [ Pass Timeout ]