This part changes the signatures for various finalization API to take
not JSContext* but rather either JSFreeOp structure or its
library-private counterpart FreeOp. These structures wrap parameters
that are passed to the finalizers removing most of explicit dependencies
on JSContext in the finalization code.
Currently the GC finalizes on the background thread only objects with null
JSClass::finalize. However, this implies that any object that uses
JS_FinalizeStub for the finalizer would be prevented from the background
finalization.
To fix this the patch removes JS_FinalizeStub replacing it with NULL in all
cases when the class has no custom finalizer. For style consistency the patch
also removed the usage of JSCLASS_NO_OPTIONAL_MEMBERS in the static
declarations as the compiler fills the missing fields with null in any cases.
