[Telemetry] Disable tab_switching benchmark on android.

Benchmark was acidentally re-enable on android even though
it is meant to be a desktop only benchmark.

TBR=charliea@chromium.org

Bug: 740581, 713222
Change-Id: I59548ffaa2a3a6c5528d2282f23bd16096b41f79
Reviewed-on: https://chromium-review.googlesource.com/565322Reviewed-by: Ned Nguyen <nednguyen@google.com>
Reviewed-by: rnephew <rnephew@chromium.org>
Commit-Queue: rnephew <rnephew@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485329}

tools/perf/benchmarks/tab_switching.py

@@ -50,5 +50,6 @@ class TabSwitchingTypical25(perf_benchmark.PerfBenchmark):
   def GetExpectations(self):
     class StoryExpectations(story.expectations.StoryExpectations):
       def SetExpectations(self):
-        pass # No tests disabled.
+        self.PermanentlyDisableBenchmark([story.expectations.ALL_MOBILE],
+                                         'Desktop benchmark.')
     return StoryExpectations()