Spun off from bug 791716 and a review thread at 

https://chromium-review.googlesource.com/c/chromium/src/+/807353 .

IDS_EXTENSION_PROMPT_RATING_ACCESSIBLE_TEXT is compliant to the ICU
message format spec (so, the unittest passes), but it's rejected
by the translation infra's ICU msg syntax checker because it has
additional constraints on the ICU syntax: A plural selector should be
at the outer most level.

To prevent a message (ICU syntax) compliant to the ICU msg format syntax that would be rejected by Google's translation tools from being added, we need a check (presubmit or otherwise) to do the same check as done by Google's internal tools.