Disable Telemetry minidump test on Sierra

Disables the multiple crash minidump test on Mac 10.12 Sierra. It is
currently failing 100% of the time on this OS due to only getting the
unsymbolized stack trace for the first crash.

TBR=crouleau@chromium.org

Bug: 634156
Change-Id: I0378b43cd88474156085def292ba35d13ce5680c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1695656Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676113}

tools/perf/core/minidump_unittest.py

@@ -88,7 +88,9 @@ class BrowserMinidumpTest(tab_test_case.TabTestCase):
       self.assertTrue(crash_function in sections[4])
 
   @decorators.Isolated
-  @decorators.Disabled('chromeos', 'android', 'win7')
+  # Disabled on Mac 10.12 (Sierra) due to it not getting a stack trace to
+  # symbolize from the second crash.
+  @decorators.Disabled('chromeos', 'android', 'win7', 'sierra')
   def testMultipleCrashMinidumps(self):
     # Wait for the browser to restart fully before crashing
     self._LoadPageThenWait('var cat = "dog";', 'cat')