Improve printing for Symbols.

Improve printing for Symbols.

Private symbols we create in the heap don't have names, but we can
resolve them to a constant string.

This gives handy debugger output like:

(gdb) job 0x2020c67d
0x2020c67d: [Symbol]
 - hash: 547385396
 - name: 0x20208091 <undefined> (uninitialized_symbol)
 - private: 1
 - own: 1
$7 = void
(gdb)

or with ShortPrint() in an array:

...
  [5]: 0x2020c67d <Symbol: 547385396 (uninitialized_symbol)>
...

Printing help for internal symbols

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/677633003

Cr-Commit-Position: refs/heads/master@{#24869}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24869 ce2b1a6d-e550-0410-aec6-3dcde31c8c00