JSRuntime is merged into JSContext in SpiderMonkey 52.

We reorganize things a bit to reflect the new situation. The files
runtime.cpp and runtime.h are renamed to engine.cpp and engine.h, and
will contain everything having to do with the SpiderMonkey engine:
initializing the JSContext, and any callbacks that the engine requires
embedders to set.

https://bugzilla.gnome.org/show_bug.cgi?id=784196