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

chrome/browser/android/vr_shell/fps_meter.cc

Index: chrome/browser/android/vr_shell/fps_meter.cc
diff --git a/chrome/browser/android/vr_shell/fps_meter.cc b/chrome/browser/android/vr_shell/fps_meter.cc
new file mode 100644
index 0000000000000000000000000000000000000000..09319b1815ce47fb228e3e519c49a340c3aadf92
--- /dev/null
+++ b/chrome/browser/android/vr_shell/fps_meter.cc
@@ -0,0 +1,58 @@
+// 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;
+
+}  // namepsace
+
+FPSMeter::FPSMeter() : total_time_us_(0) {
+  frame_times_.reserve(kNumFrameTimes);
+}
+
+FPSMeter::~FPSMeter() {}
+
+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) {
+    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