I'm not sure I understand what you mean by "Aren't the protocols and the execution engine the plattform on which a browser is built?". But I'll try to elaborate.
Chrome is built on the Content API. So it's a whole lot of C++ and GUI code on top of an API that lets you create a view and display web contents in it. The ExoBrowser wraps this Content API (part of it for now) into a JS API so that you can do all the GUI and specific browser code in Javascript instead of C++.
Additionally Chrome is not really an OS, Chrome OS is an "OS" (more a window system on top of Linux) based on Aura (View Management, kind of like GTK) and Chrome to run the apps.
Chrome is built on the Content API. So it's a whole lot of C++ and GUI code on top of an API that lets you create a view and display web contents in it. The ExoBrowser wraps this Content API (part of it for now) into a JS API so that you can do all the GUI and specific browser code in Javascript instead of C++.
Additionally Chrome is not really an OS, Chrome OS is an "OS" (more a window system on top of Linux) based on Aura (View Management, kind of like GTK) and Chrome to run the apps.