Add SlidingAverage to FPSMeter, use for WebVR prediction time

chrome/browser/android/vr_shell/vr_shell_gl.h

Index: chrome/browser/android/vr_shell/vr_shell_gl.h
diff --git a/chrome/browser/android/vr_shell/vr_shell_gl.h b/chrome/browser/android/vr_shell/vr_shell_gl.h
index 1b099a2c3025ba38d0fc1cb555ee8397f2e2e4e0..4c1f8c134bc6dff8e7cfc47a1e46d1f388c32055 100644
--- a/chrome/browser/android/vr_shell/vr_shell_gl.h
+++ b/chrome/browser/android/vr_shell/vr_shell_gl.h
@@ -43,6 +43,7 @@ namespace vr_shell {
 
 class FPSMeter;
 class MailboxToSurfaceBridge;
+class SlidingAverage;
 class UiElement;
 class UiScene;
 class VrBrowserInterface;
@@ -273,6 +274,9 @@ class VrShellGl : public device::mojom::VRVSyncProvider {
 
   std::unique_ptr<FPSMeter> fps_meter_;
 
+  std::unique_ptr<SlidingAverage> webvr_js_time_;
+  std::unique_ptr<SlidingAverage> webvr_render_time_;
+
   gfx::Point3F pointer_start_;
 
   base::WeakPtrFactory<VrShellGl> weak_ptr_factory_;