Tentatively goes into WebVR presentation mode if page listens to vrdisplayactivate.

device/vr/vr_service.mojom

 // Copyright 2015 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.
 
 module device.mojom;
 
 import "mojo/common/time.mojom";
 import "gpu/ipc/common/mailbox_holder.mojom";
 import "gpu/ipc/common/sync_token.mojom";
 
@@ skipping 97 matching lines @@
   // no mapping.
   GetVSync() => (VRPose? pose, mojo.common.mojom.TimeDelta time, int16 frameId,
                  Status status);
 };
 
 interface VRDisplayClient {
   OnChanged(VRDisplayInfo display);
   OnExitPresent();
   OnBlur();
   OnFocus();
-  OnActivate(VRDisplayEventReason reason);
+  OnActivate(VRDisplayEventReason reason) => (bool presentRequested);
   OnDeactivate(VRDisplayEventReason reason);
 };