[Telemetry] Disable failing story in smoothness.key_silk_cases

Test is failing because the story contains two interaction records.
Smoothness benchmarks only support one record.

TBR=nednguyen@google.com

Change-Id: Idc910eb5c8010e431f33872ed0d4328d66f55167
Reviewed-on: https://chromium-review.googlesource.com/677492Reviewed-by: rnephew <rnephew@chromium.org>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: rnephew <rnephew@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503571}

tools/perf/benchmarks/smoothness.py

@@ -261,7 +261,10 @@ class SmoothnessKeySilkCases(_Smoothness):
   def GetExpectations(self):
     class StoryExpectations(story_module.expectations.StoryExpectations):
       def SetExpectations(self):
-        pass  # Nothing
+        self.DisableStory(
+            'inbox_app.html?slide_drawer', [story_module.expectations.ALL],
+            'Story contains multiple interaction records. Not supported in '
+            'smoothness benchmarks.')
     return StoryExpectations()