Revert "Disable browse:media:tumblr on Mac 10.11 Perf"

This reverts commit b7662394.

Reason for revert: The underlying issue (crbug.com/760886) has been fixed so reenable

Original change's description:
> Disable browse:media:tumblr on Mac 10.11 Perf
> 
> TBR=nednguyen@chromium.org,altimin@chromium.org
> 
> Bug: 760966
> Change-Id: If6eb9c9f865925df89ef3df74dd49dd179468b47
> Reviewed-on: https://chromium-review.googlesource.com/648248
> Reviewed-by: Charlie Andrews <charliea@chromium.org>
> Reviewed-by: Alexander Timin <altimin@chromium.org>
> Commit-Queue: Charlie Andrews <charliea@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#499324}

TBR=nednguyen@google.com,charliea@chromium.org,altimin@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 760966
Change-Id: I031b6d73da8f31b9fd10263b67671083409dd2d1
Reviewed-on: https://chromium-review.googlesource.com/726839
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509916}

tools/perf/page_sets/system_health/expectations.py

@@ -18,8 +18,6 @@ class SystemHealthDesktopCommonExpectations(expectations.StoryExpectations):
                       'crbug.com/649392')
     self.DisableStory('play:media:pandora', [expectations.ALL],
                       'crbug.com/64939')
-    self.DisableStory('browse:media:tumblr',
-                      [expectations.MAC_10_11], 'crbug.com/760966')
     self.DisableStory('browse:media:tumblr',
                       [expectations.ALL_WIN], 'crbug.com/773393')
     self.DisableStory('browse:news:cnn',
@@ -45,8 +43,6 @@ class SystemHealthDesktopMemoryExpectations(expectations.StoryExpectations):
                       'crbug.com/649392')
     self.DisableStory('play:media:pandora', [expectations.ALL],
                       'crbug.com/64939')
-    self.DisableStory('browse:media:tumblr',
-                      [expectations.MAC_10_11], 'crbug.com/760966')
     self.DisableStory('browse:news:cnn',
                       [expectations.ALL_MAC], 'crbug.com/728576')
     self.DisableStory('browse:social:twitter_infinite_scroll',