A unity build can be much faster (8x-13x faster in the compilation phase) and content takes a long time to build so this is tracking an experiment to make it compile in a unity build.

Complications:
* X11 headers leak defines -> Fixed with undef and X11Constants class
* libyuv, webrtc leak identical FOURCC define that collides -> #undef
* libyuv, webrtc, leveldatabase leak identical threading macros -> #undef
* IPC headers -> wrapper files
* content/common visibility -> ??
* Name collisions -> Renames or share code instead of duplicating it