SM52 essentially replaces JSRuntime with JSContext (though JSContext
itself was replaced with JSCompartment).
To prepare for this migration, rename all Runtime-related things to
Context.
Part of the SM52 migration, stage: SM45 compatible.
Patch by: Itms
Refs #4893
Differential Revision: https://code.wildfiregames.com/D3091
This was SVN commit r24181.
wraitii