Version: Chromium experimental WebVR branch 54.0.2803.0. Built from sources on 2016-07-22.
OS: Windows 10

What steps will reproduce the problem?
(1) Set v-sync to “On” instead of the default “Application-controlled” from NVIDIA control panel 3D settings.
(2) Run Chromium built from the experimental WebVR branch.
(3) Run https://toji.github.io/webvr-samples/03-vr-presentation.html on a HMD, either Vive or Oculus.

What is the expected output?
WebVR runs at the HMD FPS, usually 90 FPS.

What do you see instead?
WebVR runs at most at the monitor refresh rate, usually 60 FPS.

Other VR apps are still able to run 90 FPS with v-sync on in the control panel, so it must be something that Chrome is specifically doing that prevents it from running at 90 FPS in this case.