Currently WebUIPolymer2 flag is turned on by default on ToT for Desktop, and off by default for ChromeOS.

The list of known regressions is tracked by issue 896748, and it is unclear yet whether all of them will be fixed on time for M72 branch cut.

@markchang: I am thinking of having the flag on 50% on Canary/Dev, and 0% for Beta/Stable for now.