When it comes to Google's products and services, they all do seem to go through very frequent changing and evolving. Chrome OS is no exception and neither is the Chrome app launcher or even the actual Chrome app launcher icon and especially on Chrome OS.
In terms of the Chrome app launcher in general, this is an aspect which was recently removed completely from non-Chrome OS devices, making it now a staple of only Chrome OS-running products like Chromebooks. However, what is interesting is that Google seems to be continually changing the actual icon, which denotes the launcher in the bottom left-hand corner of the screen. At first, the icon used to appear as a small grid, which when clicked opened up the launcher and provided access to the various installed or accessible Chrome apps. However, back in February of this year, it started being noticed that the grid icon had been replaced by a magnifying glass icon. Fast forward and that same magnifying glass icon now seems to be being replaced by a new circular icon.
At present, the change is only being noted on the developer and beta channels of Chrome OS, so if you are not on either of those channels, you are unlikely to be seeing the new circular icon yet. Although, it does stand to reason that you will soon enough when the change begins to roll out to the stable version of Chrome OS as well. What is interesting though and as +AnjoCerdeña on G+ (one of those who first spotted the change) points out, the new icon does very much resemble the home button icon on Android. Which does naturally lead to the assumption that Google might be looking to harmonize the look of the home button on Android with that of the app launcher (also a home button of sorts) on Chrome OS. Thinking which would fit in nicely with the recent developments in Chrome OS, like the adding of support for Android apps. Either way, if you are on one of the less-stable versions of Chrome OS, you should start to notice the new Android-like icon being used for the Chrome OS app launcher icon soon enough, if you have not already that is.