Rarely (about once a day on the bots), we'll get a DCHECK failure:

[FATAL:gvr_scheduler_delegate.cc(622)] Check failed: !gpu_trace_ || gpu_trace_->fence()->HasCompleted().

The stack doesn't seem to symbolize correctly, so not sure what's going on under the hood. However, base on the timing from logcat, this appears to occur during VR browser entry, e.g.

10-29 07:58:32.378   918 14016 I ActivityManager: START u0 {cat=[com.google.intent.category.DAYDREAM] flg=0x14010000 cmp=org.chromium.chrome/.browser.ChromeTabbedActivity} from uid 10122

10-29 07:58:33.265 17226 17533 F chromium: [FATAL:gvr_scheduler_delegate.cc(622)] Check failed: !gpu_trace_ || gpu_trace_->fence()->HasCompleted(). 

The tests that I've seen hit this so far on a Pixel XL w/ O are:
VrBrowserNavigationTest#testWebXrFullscreenTo2d
VrBrowserNavigationTest#testRendererKilledInFullscreenStaysInVr
VrBrowserControllerInputTest#testDragRefresh
VrBrowserNativeUiTest#testOmniboxContainsCurrentUrl