Add SlidingAverage to FPSMeter, use for WebVR prediction time

chrome/browser/android/vr_shell/fps_meter_unittest.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 "base/macros.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace vr_shell {
@@ skipping 59 matching lines @@
   EXPECT_FLOAT_EQ(0.0, meter.GetFPS());
 
   for (int i = 0; i < 5; ++i) {
     now += frame_time;
     meter.AddFrame(now);
     EXPECT_TRUE(meter.CanComputeFPS());
     EXPECT_NEAR(1.0 / frame_time.InSecondsF(), meter.GetFPS(), kTolerance);
   }
 }
 
+TEST(SlidingAverage, Basics) {
+  SlidingAverage meter(5);
+
+  // No values yet
+  EXPECT_EQ(42, meter.GetAverageOrDefault(42));
+  EXPECT_EQ(0, meter.GetAverage());
+
+  meter.AddSample(100);
+  EXPECT_EQ(100, meter.GetAverageOrDefault(42));
+  EXPECT_EQ(100, meter.GetAverage());
+
+  meter.AddSample(200);
+  EXPECT_EQ(150, meter.GetAverage());
+
+  meter.AddSample(10);
+  EXPECT_EQ(103, meter.GetAverage());
+
+  meter.AddSample(10);
+  EXPECT_EQ(80, meter.GetAverage());
+
+  meter.AddSample(10);
+  EXPECT_EQ(66, meter.GetAverage());
+
+  meter.AddSample(10);
+  EXPECT_EQ(48, meter.GetAverage());
+
+  meter.AddSample(10);
+  EXPECT_EQ(10, meter.GetAverage());
+
+  meter.AddSample(110);
+  EXPECT_EQ(30, meter.GetAverage());
+}
+
 }  // namespace vr_shell