[PowerProfiler] Power Profiler service should detect the sampling rate from the data provider.

content/browser/power_profiler/power_data_provider.h

 // Copyright 2014 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.
 
 #ifndef CONTENT_BROWSER_POWER_PROFILER_POWER_DATA_PROVIDER_H_
 #define CONTENT_BROWSER_POWER_PROFILER_POWER_DATA_PROVIDER_H_
 
 #include <vector>
 
 #include "base/memory/scoped_ptr.h"
 #include "content/browser/power_profiler/power_event.h"
 
+namespace base {
+class TimeDelta;
+}  // namespace base
+
 namespace content {
 
 typedef std::vector<PowerEvent> PowerEventVector;
 
 // A class used to get power usage.
 class PowerDataProvider {
  public:
   static scoped_ptr<PowerDataProvider> Create();
 
   PowerDataProvider() {}
   virtual ~PowerDataProvider() {}
 
   // Returns a vector of power events, one per type, for the types it supports.
   virtual PowerEventVector GetData() = 0;
+
+  // Returns sampling rate at which the provider can operate.
+  virtual base::TimeDelta GetSamplingRate() = 0;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_POWER_PROFILER_POWER_DATA_PROVIDER_H_