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

chrome/browser/android/vr_shell/fps_meter.cc

 // 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.
 
 #include "chrome/browser/android/vr_shell/fps_meter.h"
 
 #include <algorithm>
 
 namespace vr_shell {
 
 namespace {
 
-static constexpr size_t kNumFrameTimes = 200;
+static constexpr size_t kNumFrameTimes = 10;
 
 }  // namepsace
 
 FPSMeter::FPSMeter() : total_time_us_(0) {
   frame_times_.reserve(kNumFrameTimes);
 }
 
 FPSMeter::~FPSMeter() {}
 
+size_t FPSMeter::GetNumFrameTimes() {
+  return kNumFrameTimes;
+}
+
 void FPSMeter::AddFrame(const base::TimeTicks& time_stamp) {
   if (last_time_stamp_.is_null()) {
     last_time_stamp_ = time_stamp;
     return;
   }
 
   base::TimeDelta delta = time_stamp - last_time_stamp_;
   last_time_stamp_ = time_stamp;
 
   total_time_us_ += delta.InMicroseconds();
 
-  if (frame_times_.size() + 1 < kNumFrameTimes) {
+  if (frame_times_.size() < kNumFrameTimes) {
     frame_times_.push_back(delta);
   } else {
     total_time_us_ -= frame_times_[current_index_].InMicroseconds();
     frame_times_[current_index_] = delta;
   }
 
   current_index_++;
   if (current_index_ >= kNumFrameTimes)
     current_index_ = 0;
 }
 
 bool FPSMeter::CanComputeFPS() const {
   return !frame_times_.empty();
 }
 
 // Simply takes the average time delta.
 double FPSMeter::GetFPS() const {
   if (!CanComputeFPS())
     return 0.0;
 
   return (frame_times_.size() * 1.0e6) / total_time_us_;
 }
 
 }  // namespace vr_shell