Re-adjust test expectations for fast/dom/timer-throttling-hidden-page.html

Its test expectation was changed recently by https://crrev.com/c/1642031 but it looks it just keeps failing on Mac.

Sample failures:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/WebKit%20Mac10.13%20%28retina%29/15149
http://build.chromium.org/p/chromium.mac/builders/Mac10.12%20Tests/builds/23650
http://build.chromium.org/p/chromium.mac/builders/Mac10.12%20Tests/builds/23644
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.13%20Tests/14224

TBR=szager@chromium.org

Bug: 971031
Change-Id: Icf49dcfaaa6492d36725fd62b3e5e59ee904f884
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1644913
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666175}

third_party/blink/web_tests/TestExpectations

@@ -5883,3 +5883,7 @@ crbug.com/970135 [ Mac ] virtual/focusless-spat-nav/fast/spatial-navigation/focu
 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 ]
+
+# Sheriff 2019-06-05
+crbug.com/971031 [ Mac ] fast/dom/timer-throttling-hidden-page.html [ Pass Failure ]
+