Chrome Version:67.0.3396.8
OS: Android O
VRCore: 1.14.190701955
Device: Pixel 2 XL

What steps will reproduce the problem?
(1) Enter VR browsing
(2) go to https://webvr.info/samples/03-vr-presentation.html
(3) press Enter VR button to start presentation.
(4) press controller's app button to return to VR browsing.
(5) repeat entering and exiting presentation to VR browsing.

Additional repro steps:
(1) Enter VR browsing
(2) Click on URL bar to open Omnibox and keyboard entry.
(3) press controller's app button to return to VR browsing.
(4) repeat entering and exiting Omnibox text entry mode.

What is the expected result? No screen flashes.

What happens instead? A noticeable screen flash frequently occurs. See video.