The reason of failure is likely (1) the system preference for
"always show toolbar in full screen" may be different on different bots.

While checking the bug, I noticed there could be other failure due to
(2) fullscreen state change is done asynchronously. When we check whether the browser is in fullscreen such as browser_view->IsInFullscreen(), it ends up checking Widget::IsFullscreen(). On Mac, it goes to BridgedNativeWidget::target_fullscreen_state_ which is only set upon receiving windowWillEnterFullScreen/windowWillExitFullScreen notifications. 

Although (2) is unlikely to be the reason caused failure on the specific bot in #1, it may result in failure in some cases. So it needs to be addressed also. 

Looks like (2) is not really a concern, the fix was landed https://chromium-review.googlesource.com/c/chromium/src/+/1111015