HipChat is a group messaging system that has been on the American market for several years and is used by quite a lot of people. Looking at image above you will notice that between the icons Messages and the HipChat icon there are many more than similar elements. Basically, Apple took the HipChat icon, slightly changed the order of the two bubbles, worked a little on the colors and included some new elements, then put it as the main icon for the future Messages application.
I don't know if Apple had this icon designed before HipChat appeared, but if it didn't and HipChat registered the design in the US and other countries around the world, then Apple either paid to use the icon or is going to pay for the right to use its elements.
