WebVR supported two variants for synchronizing cross-process image transfer via mailbox. The intended path was to defer waiting for transfer confirmation until the next frame to avoid blocking the Javascript thread unnecessarily, but this was causing flickering (black frames), so the default was to use the slower path with waiting after submitting.

Now that the underlying synchronization is fixed (see  issue 723962  and crrev.com/2891033002 / r473687), we want to use the faster path by default.