This is the exact sort of thing anti-monopoly laws are intended to work against: that a major market force in one market uses that power to intrude or support itself in another market. Since Google Chrome undoubtedly has a sufficiently large market share that this may be a concern and the removal of this extension certainly affects the advertisement market, this seems like a very dangerous move for Google.
Yes, it may be considered “self-defense” but is the cost incurred really so large that they want to risk another lawsuit?