Stop bad results on first memory benchmark run

tools/perf/benchmarks/system_health.py

Index: tools/perf/benchmarks/system_health.py
diff --git a/tools/perf/benchmarks/system_health.py b/tools/perf/benchmarks/system_health.py
index 80539198fbdae81ca5f10c974cd12e7aa9ab8b62..75a688132d0dc67786a4f7aebe94736de9c0540e 100644
--- a/tools/perf/benchmarks/system_health.py
+++ b/tools/perf/benchmarks/system_health.py
@@ -105,6 +105,15 @@ class _MemorySystemHealthBenchmark(perf_benchmark.PerfBenchmark):
     return page_sets.SystemHealthStorySet(platform=self.PLATFORM,
                                           take_memory_measurement=True)
 
+  def SetExtraBrowserOptions(self, options):
+    # Just before we measure memory we flush the system caches
+    # unfortunately this doesn't immediately take effect, instead
+    # the next story run is effected. Due to this the first story run
+    # has anomalous results. This option causes us to flush caches
+    # each time before Chrome starts so we effect even the first story
+    # - avoiding the bug.
+    options.clear_sytem_cache_for_browser_and_profile_on_start = True
+
   @classmethod
   def ShouldTearDownStateAfterEachStoryRun(cls):
     return True