We had a linker problem with a C++ - compiler that inlined process_grey_object at all uses but removed the symbol and the actual implementation.

As there are explicit instantiations of this method in concurrentMark.cpp, this probably should not happen and can be considered of a bug of the c++ compiler.
Though, it seems to be pointless to mark that method inline if there are explicit instantiations.

However, considering performance, it would be better to move the implementation of process_grey_object to concurrentMark.inline.hpp in order to inline the method in other compilation units.