VM.native_memory should report size of shareable memory

The current output looks like this:

$ jcmd <pid> native_memory
...
Total: reserved=515598KB, committed=72314KB
...
- Shared class space (reserved=25472KB, committed=25472KB)
                            (mmap: reserved=25472KB, committed=25472KB)
...

Of the "Shared class space", about 50% is mapped read-only, so it can be shared across processes. It would be useful to change the above output to

Total: reserved=515598KB, committed=72314KB, shareable=12345KB
...
- Shared class space (reserved=25472KB, committed=25472KB, shareable=12345KB)
                            (mmap: reserved=25472KB, committed=25472KB, shareable=12345KB)