Fix off-by-one in fps_meter, fix unit test to catch this.

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;
 
+  // Get sliding window size for tests.
+  size_t GetNumFrameTimes();
+
  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_