I even suspect the W3Schools % is over-estimated, as the types of users visiting the sites are typically web-developers who prefer Chrome for the superior development tooling. When you start to look at business users and non-technical users IE/Safari (on respective platforms) do much better.
If Chrome was a monopoly then doing what you're talking about MIGHT be an anti-trust violation, but that is a complex legal question (for example, everyone remembers IE as an anti-trust violation for Microsoft in the 1990s, but what about all of the other things they pushed which weren't e.g. MSN, Hotmail, Office, etc). Google Music might NOT be because the court has to determine if Google is leveraging their position to give their other products an "unfair" advantage. If Chrome started blocking other music services (or redirecting from them to Google Music) then that would 100% be an anti-trust violation if Google were a monopoly, but an advert? That's for a court to decide, shades of grey...