Encode frame number in pixel data for pose sync

device/vr/vr_service.mojom

Index: device/vr/vr_service.mojom
diff --git a/device/vr/vr_service.mojom b/device/vr/vr_service.mojom
index 0894f23facc79abd8f519ae66a7a4deea5fa7633..447f95d5adfac1862d9281cae204e4237eb96e35 100644
--- a/device/vr/vr_service.mojom
+++ b/device/vr/vr_service.mojom
@@ -13,7 +13,8 @@ struct VRFieldOfView {
 };
 
 // A display's position, orientation, velocity, and acceleration state at the
-// given timestamp.
+// given timestamp. The poseNum is a sequential ID that's incremented on
+// each distinct getPose result, it may wrap around for long sessions.
 struct VRPose {
   double timestamp;
   array<float, 4>? orientation;
@@ -22,6 +23,7 @@ struct VRPose {
   array<float, 3>? linearVelocity;
   array<float, 3>? angularAcceleration;
   array<float, 3>? linearAcceleration;
+  uint32 poseNum;
 };
 
 struct VRDisplayCapabilities {