Mark [virtual/audio-service/]media/controls/volume-slider.html flaky

The audio-service version was already marked flaky on Mac, but it's
flaky on other platforms too. The non-virtual version is also flaky
everywhere.

TBR=mlamouri@google.com

Bug: 1000051
Change-Id: I85f90e83e12169d5cd15b033e13b8d06b8149fb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1816549Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698476}

third_party/blink/web_tests/TestExpectations

@@ -6169,7 +6169,8 @@ crbug.com/919204 external/wpt/scroll-to-text-fragment/scroll-to-text-fragment.ht
 
 crbug.com/999209 virtual/lazyload-image/http/tests/lazyload/style-dimension.html [ Timeout ]
 
-crbug.com/1000051 [ Mac ] virtual/audio-service/media/controls/volume-slider.html [ Failure Timeout Pass ]
+crbug.com/1000051 media/controls/volume-slider.html [ Failure Timeout Pass ]
+crbug.com/1000051 virtual/audio-service/media/controls/volume-slider.html [ Failure Timeout Pass ]
 
 # Sheriff 2019-09-03
 crbug.com/999799 [ Mac Linux ] external/wpt/html/rendering/dimension-attributes.html [ Pass Timeout ]