Mark fast/dom/timer-throttling-hidden-page.html as flaky on Mac

Bug: 971031
Change-Id: I3c6fa661fca7830e58a2903b2d72a5295ecb6d49
No-Try: True
Tbr: szager@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212235Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771451}

third_party/blink/web_tests/TestExpectations

@@ -5885,3 +5885,5 @@ crbug.com/1080609 virtual/threaded/external/wpt/scroll-animations/element-based-
 
 # Fails when test moved to use full compositor pipe.
 crbug.com/1085832 [ Fuchsia ] images/size-failure.html [ Timeout ]
+
+crbug.com/971031 [ Mac ] fast/dom/timer-throttling-hidden-page.html [ Pass Failure ]
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