Four Chromebooks now support trackpad pinch-to-zoom and this includes the new Google Pixelbook, the Chromebook Pixel from 2015, the Samsung Chromebook Plus, and the Samsung Chromebook Pro, with a possibility that more Chromebooks could eventually support this new feature sometime in the near future. That said there is no evidence right now that other existing Chromebooks will get to make use of this anytime soon and if Google does decide to implement it on more machines sooner rather than later, there's no indication for the time being on what models Google would choose to integrate it into next.
For those that have one of the four above-mentioned Chromebooks using the pinch-to-zoom is pretty easy and straightforward. Simply open up Chrome and while browsing any webpage, pinch the trackpad in or out just like you would on your Android smartphone or tablet and the browser view will zoom in or out based on how you're pinching the trackpad. This is a simple little addition and by no means a major new change for the operating system, but if you do use Chrome OS a lot then chances are you have been hoping for an easier way to zoom in and zoom out. In Chrome on the desktop, this is typically handled by clicking on the overflow menu to the far right of the address bar, and from there you'll see an option for zooming in and out by clicking the little "+" and "-" icons. This isn't convenient at all, so it shouldn't be a surprise that Google wanted to add a method which allowed users to zoom in and out on content much easier.
According to the report this is something that has been workable on the Pixelbook and the other Chromebooks mentioned here since around August, so this isn't technically an entirely new feature, but it is possible that it may have been in early testing back then or that it just went unnoticed with Google not having mentioned it as a usable feature for those models. Whatever the case, pinch-to-zoom is up and running now on the Chromebooks listed above, so if you own one or more of them then you're now able to zoom in and out a lot more easily than before.