Revert of perf: Disable browse:news:reddit on Mac (patchset #1 id:1 of...

Revert of perf: Disable browse:news:reddit on Mac (patchset #1 id:1 of https://codereview.chromium.org/2484783002/ )

Reason for revert:
The failure does not reproduce at TOT locally.
Re-enabling to see if it still fails on bots.

Original issue's description:
> perf: Disable browse:news:reddit on Mac
>
> BUG=662959
> TBR=perezju@chromium.org
>
> Committed: https://crrev.com/31caf2b1bb934e61b2b79fda4ebb8a8791546f54
> Cr-Commit-Position: refs/heads/master@{#430320}

TBR=perezju@chromium.org,skyostil@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=662959

Review-Url: https://codereview.chromium.org/2565393002
Cr-Commit-Position: refs/heads/master@{#437869}

tools/perf/page_sets/system_health/browsing_stories.py

@@ -176,7 +176,6 @@ class QqMobileStory(_NewsBrowsingStory):
   SUPPORTED_PLATFORMS = platforms.MOBILE_ONLY
 
 
-@decorators.Disabled('mac')  # crbug.com/662959
 class RedditDesktopStory(_NewsBrowsingStory):
   """The top website in http://www.alexa.com/topsites/category/News"""
   NAME = 'browse:news:reddit'