It more or less is, from a legal stand point, or atleast when referring to anti-trust laws.
That doesn't mean that they are doing anti-competitive stuff and are viable for enforcement, being a monopoly in an open market is not illegal, it does mean that with that level of marketshare they have some legal limitations that their competitors don't have. Usually it's a limit on merges, acquisitions, price setting and bundling.
In the EU you don't even need market dominance to break anti-trust law, while in the US you do. In either case, Chrome is not committing anti-competitive behavior nor is it the only option on the market, hence it is not a monopoly.
Being a popular product in a market full of consumer options is not a monopoly. If Chrome prevented you from downloading other browsers, then that would be monopolistic behavior, but Chrome is not doing that.
That doesn't mean that they are doing anti-competitive stuff and are viable for enforcement, being a monopoly in an open market is not illegal, it does mean that with that level of marketshare they have some legal limitations that their competitors don't have. Usually it's a limit on merges, acquisitions, price setting and bundling.