I fixed a couple of other problems with webrtc and it now builds. Tom invited me to post patches to him so that these fixes can get into the main release. I look forward to doing that, it's been a while since I donated patches anywhere. Unfortunately, my 8Gb SD card has filled up and, despite deleting various things, I need to create another one. Luckily Jon Masters has posted a clean 'how to' install Fedora (complete with lurid warnings) @ https://fedoraproject.org/wiki/Architectures/ARM/Samsung_Chromebook_2012.