[vr] Add a frame rate counter for the vr UI

chrome/browser/android/vr_shell/fps_meter.h

+// Copyright 2017 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 CHROME_BROWSER_ANDROID_VR_SHELL_FPS_METER_H_
+#define CHROME_BROWSER_ANDROID_VR_SHELL_FPS_METER_H_
+
+#include <vector>
+
+#include "base/macros.h"
+#include "base/time/time.h"
+
+namespace vr_shell {
+
+// Computes fps based on submitted frame times.
+class FPSMeter {
+ public:
+  FPSMeter();
+  ~FPSMeter();
+
+  void AddFrame(const base::TimeTicks& time_stamp);
+
+  bool CanComputeFPS() const;
+
+  double GetFPS() const;
+
+ private:
+  size_t current_index_;
+  int64_t total_time_us_;
+  base::TimeTicks last_time_stamp_;
+  std::vector<base::TimeDelta> frame_times_;
+  DISALLOW_COPY_AND_ASSIGN(FPSMeter);
+};
+
+}  // namespace vr_shell
+
+#endif  // CHROME_BROWSER_ANDROID_VR_SHELL_FPS_METER_H_