Alon is the expert on the C++ conversion. The compilation tool he uses is Emscripten which has a command line compiler similar to gcc. In his half of the presentation, he demonstrated a full 3D game running in the browser, ported from the C++ code with only a few minor tweaks. The original code was written in C++/OpenGL ES 2.0. In fact, you can play the game in your browser here.
Because of the current limits of WebGL, Chrome and Firefox work automatically because WebGL is enabled in those browsers. For Safari and Opera, you’ll need to turn on WebGL. For IE, Microsoft has not committed to support the WebGL standard.
In both cases, some optimization was needed, but not much. There are a few minor limitations (like no DirectX conversion available for C++). In addition, you’ll see a performance loss of between 1.5x to 6x depending on what kinds of code you write (although this is improving with both speakers expecting to see significant increases on the browser side by mid-2013.
The slides from the presentation can be found at http://jsil.org/GDCOnline2012.