Animated PNG files are essentially like GIF files without the limitations; they can have a full 24 bit color spectrum, and support 8 bit transparency. Support for them has long been a part of other browsers like Opera and Firefox. The feature finally making it into Chrome is a long time coming. Requests to support animated PNG files natively in Chrome have been going around since about 2008, but Google has not gotten around to putting the feature in until now. Per a conversation over the issue back in 2013, most of the problem revolved around the fact that the libpng code could not simply be patched to add in support for the feature, but would have to be modified from the system level on up.
Animated PNG support is just the latest in a long line of nifty features that have been hitting the Chrome and Chromium projects via a storm of updates in the last few months. On top of new features, improvements to things like download handling and translations have been going into the latest Chrome commits, as well. Chrome 59 is still largely in its infancy, since Chrome 58 just hit the Stable channel earlier this month, so even more new features could end up going into the project before the newest version makes its way to the Stable channel.