//tools/perf: Disable smoothness benchmark story on mac

The story seems to be failing, so disabling it on mac for now.

TBR=rnephew

Bug: 770904
Change-Id: I5516cd57f726361e09ac976fdc32da1bd87abcfa
Reviewed-on: https://chromium-review.googlesource.com/696283Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505861}

tools/perf/benchmarks/smoothness.py

@@ -182,6 +182,9 @@ class SmoothnessKeyDesktopMoveCases(_Smoothness):
         self.DisableStory('https://mail.google.com/mail/',
                           [story_module.expectations.ALL_WIN],
                           'crbug.com/750131')
+        self.DisableStory('https://mail.google.com/mail/',
+                          [story_module.expectations.ALL_MAC],
+                          'crbug.com/770904')
     return StoryExpectations()