[Telemetry] Add the ability to flush the system file cache to platform.

tools/telemetry/telemetry/page/page_measurement_value.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
-from telemetry.page import perf_tests_helper
+from telemetry.core import perf_tests_helper
 
 def _Mean(l):
   return float(sum(l)) / len(l) if len(l) > 0 else 0.0
 
 class PageMeasurementValue(object):
   def __init__(self, trace_name, units, value, chart_name, data_type):
     self.trace_name = trace_name
     self.units = units
     self.value = value
     self.chart_name = chart_name
     self.data_type = data_type
 
   @property
   def measurement_name(self):
     if self.chart_name:
       return '%s.%s' % (self.chart_name, self.trace_name)
     else:
       return self.trace_name
 
   @property
   def output_value(self):
     if 'histogram' in self.data_type:
       (mean, _) = perf_tests_helper.GeomMeanAndStdDevFromHistogram(self.value)
       return mean
     elif isinstance(self.value, list):
       return _Mean(self.value)
     else:
       return self.value