Add SlidingAverage to FPSMeter, use for WebVR prediction time

chrome/browser/android/vr_shell/fps_meter.h

Index: chrome/browser/android/vr_shell/fps_meter.h
diff --git a/chrome/browser/android/vr_shell/fps_meter.h b/chrome/browser/android/vr_shell/fps_meter.h
index c52f4100ee599367977f2f1066de461f181be81c..86da9763d185375248271f2d3281c8d2a186c5f8 100644
--- a/chrome/browser/android/vr_shell/fps_meter.h
+++ b/chrome/browser/android/vr_shell/fps_meter.h
@@ -12,10 +12,32 @@
 
 namespace vr_shell {
 
+class SampleQueue {
+ public:
+  explicit SampleQueue(size_t window_size);
+  ~SampleQueue();
+
+  int64_t GetSum() const { return sum_; }
+
+  void AddSample(int64_t value);
+
+  size_t GetCount() const { return samples_.size(); }
+
+  // Get sliding window size for tests.
+  size_t GetWindowSize() const { return window_size_; }
+
+ private:
+  int64_t sum_ = 0;
+  size_t current_index_ = 0;
+  size_t window_size_;
+  std::vector<int64_t> samples_;
+};
+
 // Computes fps based on submitted frame times.
 class FPSMeter {
  public:
   FPSMeter();
+  explicit FPSMeter(size_t window_size);
   ~FPSMeter();
 
   void AddFrame(const base::TimeTicks& time_stamp);
@@ -28,13 +50,24 @@ class FPSMeter {
   size_t GetNumFrameTimes();
 
  private:
-  size_t current_index_;
-  int64_t total_time_us_;
+  SampleQueue frame_times_;
   base::TimeTicks last_time_stamp_;
-  std::vector<base::TimeDelta> frame_times_;
   DISALLOW_COPY_AND_ASSIGN(FPSMeter);
 };
 
+class SlidingAverage {
+ public:
+  explicit SlidingAverage(size_t window_size);
+  ~SlidingAverage();
+
+  void AddSample(int64_t value);
+  int64_t GetAverageOrDefault(int64_t default_value) const;
+  int64_t GetAverage() const { return GetAverageOrDefault(0); }
+
+ private:
+  SampleQueue values_;
+};
+
 }  // namespace vr_shell
 
 #endif  // CHROME_BROWSER_ANDROID_VR_SHELL_FPS_METER_H_