Add SlidingAverage to FPSMeter, use for WebVR prediction time

chrome/browser/android/vr_shell/vr_shell_gl.cc

Index: chrome/browser/android/vr_shell/vr_shell_gl.cc
diff --git a/chrome/browser/android/vr_shell/vr_shell_gl.cc b/chrome/browser/android/vr_shell/vr_shell_gl.cc
index 14a22becd74bfc2730231dd912f501a23375bc54..6bd8fd32cc178bf762c018f687976a7c0949c58e 100644
--- a/chrome/browser/android/vr_shell/vr_shell_gl.cc
+++ b/chrome/browser/android/vr_shell/vr_shell_gl.cc
@@ -82,6 +82,10 @@ static constexpr int kViewportListHeadlockedOffset = 2;
 // 2-3 frames.
 static constexpr unsigned kPoseRingBufferSize = 8;
 
+// Number of frames to use for sliding averages for pose timings,
+// as used for estimating prediction times.
+static constexpr unsigned kWebVRSlidingAverageSize = 5;
+
 // Criteria for considering holding the app button in combination with
 // controller movement as a gesture.
 static constexpr float kMinAppButtonGestureAngleRad = 0.25;
@@ -226,6 +230,8 @@ VrShellGl::VrShellGl(VrBrowserInterface* browser,
 #if DCHECK_IS_ON()
       fps_meter_(new FPSMeter()),
 #endif
+      webvr_js_time_(new SlidingAverage(kWebVRSlidingAverageSize)),
+      webvr_render_time_(new SlidingAverage(kWebVRSlidingAverageSize)),
       weak_ptr_factory_(this) {
   GvrInit(gvr_api);
 }
@@ -1524,9 +1530,20 @@ void VrShellGl::SendVSync(base::TimeDelta time,
 
   TRACE_EVENT1("input", "VrShellGl::SendVSync", "frame", frame_index);
 
+  int64_t frame_time_micros = vsync_interval_.InMicroseconds();

[cjgrant, 2017/05/24 13:37:30]

How do you feel about dropping this block into
VrShellGl::GetPredictedFrameTime() or similar?

[klausw, 2017/05/24 16:32:23]

Done, I've also added TRACE_COUNTER output so that these (+ FPS) get graphed in
event traces. 

+  // If we aim to submit at vsync, that frame will start scanning out
+  // one vsync later. Add a half frame to split the difference between
+  // left and right eye.
+  int64_t overhead_micros = frame_time_micros * 3 / 2;
+  int64_t expected_frame_time =
+      webvr_js_time_->GetAverageOrDefault(frame_time_micros) +
+      webvr_render_time_->GetAverageOrDefault(frame_time_micros);
+  int64_t prediction_nanos = (expected_frame_time + overhead_micros) * 1000;
+
   vr::Mat4f head_mat;
   device::mojom::VRPosePtr pose =
-      device::GvrDelegate::GetVRPosePtrWithNeckModel(gvr_api_.get(), &head_mat);
+      device::GvrDelegate::GetVRPosePtrWithNeckModel(gvr_api_.get(), &head_mat,
+                                                     prediction_nanos);
 
   webvr_head_pose_[frame_index % kPoseRingBufferSize] = head_mat;