Disable flaky http/tests/devtools/elements/styles-4/styles-update-from-js.js on Mac

TBR=chenwilliam@chroimum.org

Bug: 794869
Change-Id: Ifd17f87b95a0ebd2977d9cdb24608b32fe9c449a
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/829133Reviewed-by: Henrik Grunell <grunell@chromium.org>
Commit-Queue: Henrik Grunell <grunell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524364}

third_party/WebKit/LayoutTests/TestExpectations

@@ -1054,7 +1054,7 @@ crbug.com/498539 http/tests/devtools/tracing/timeline-misc/timeline-bound-functi
 crbug.com/498539 virtual/threaded/http/tests/devtools/tracing/timeline-misc/timeline-bound-function.js [ Pass Failure ]
 crbug.com/498539 [ Mac ] http/tests/devtools/sources/debugger/live-edit-no-reveal.js [ Crash Pass Timeout Failure ]
 
-crbug.com/498539 crbug.com/794869 [ Win7 ] http/tests/devtools/elements/styles-4/styles-update-from-js.js [ Crash Pass Failure ]
+crbug.com/498539 crbug.com/794869 [ Win7 Mac ] http/tests/devtools/elements/styles-4/styles-update-from-js.js [ Crash Pass Failure ]
 
 crbug.com/487281 [ Mac ] fast/forms/select/menulist-narrow-width.html [ Failure ]