It's mostly C compatible, but it looks like it gets confused by a few things. I tried formatting content/common/frame.mojom, and a few odd looking lines:

This one is probably OK:
// See src/content/common/navigation_params.h                                                  [Native] struct CommonNavigationParams;

This one is probably not:
// An opaque handle that keeps alive the associated render process even after                  
// the frame is detached. Used by resource requests with "keepalive" specified.                
interface KeepAliveHandle{};

And this one is definitely not:
interface FrameFactory {                       
  CreateFrame(int32 frame_routing_id, Frame & frame);                                          
};