Is there a rule in the style guide that covers this? Both spellings are valid, although one or the other may have greater prevalence in certain geographic regions.

I don't know that there's a Google style guide declaration for grey vs. gray. This recently came up in the MacViews work, which is why it's visible to me now (tapted@ is from Australia, which is why he started to name things "grey"). That goes to your point about greater prevalence in certain locales - in America it's "gray."

Since we have to settle on one or the other for the API, and we are an American company, we should be using "gray."

Here are a couple outside links:

The Yahoo! Style Guide
https://books.google.com/books?id=Z2m9AG-Y2lQC&pg=PT605&lpg=PT605&dq=google+style+guide+grey+gray&source=bl&ots=s-0MC56X4a&sig=jGgeaoViUCBMO-x9RGZ4PybGsP0&hl=en&sa=X&ved=0ahUKEwi8moTsuKfMAhVH12MKHfabCY0Q6AEIPDAF#v=onepage&q=google%20style%20guide%20grey%20gray&f=false

HBR - A Quick Guide to Avoiding Common Writing Errors
https://hbr.org/2015/07/a-quick-guide-to-avoiding-common-writing-errors

this sounds like something which should either be codified in a style guide after appropriate consensus, or left intentionally unspecified

Does someone here want to review https://chromium-review.googlesource.com/c/chromium/src/+/756878 then?

Archiving old bugs that haven't been actively assigned in over 180 days.

If you feel this issue should still be addressed, feel free to reopen it or to file a new issue. Thanks!