Add SlidingAverage to FPSMeter, use for WebVR prediction time

chrome/browser/android/vr_shell/fps_meter_unittest.cc

Index: chrome/browser/android/vr_shell/fps_meter_unittest.cc
diff --git a/chrome/browser/android/vr_shell/fps_meter_unittest.cc b/chrome/browser/android/vr_shell/fps_meter_unittest.cc
index 6335af86289f67b3ab7f32bb130dc5077f3f1e85..18403f387f0d91a86ecf629a35427aa4f2943399 100644
--- a/chrome/browser/android/vr_shell/fps_meter_unittest.cc
+++ b/chrome/browser/android/vr_shell/fps_meter_unittest.cc
@@ -77,4 +77,37 @@ TEST(FPSMeter, AccurateFPSWithHigherFramerate) {
   }
 }
 
+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