Amend PersistentValueMap:
- Use the surrounding map (instead of Traits::Impl) for weak callback.
- Provide for a fast reference to a mapped value.
- Restructure Traits to accomondate for the first point above.

[Why?] As discussed, I proceeded to replace Impl with the map.
The problem I encountered with that version is that now the
Traits class depends on itself: The weak-related methods require the
map type in their signature. But the map type includes the Traits class
and hence the Traits class method signatures depend on the specific Traits class. That
makes them practically un-derivable: While you can derive a Traits class
from another one, since the compiler now expects methods with a different
signature. To accommodate, I pulled the dispose traits into the weak traits
class. I also removed the Impl*/MapType* parameter from the Dispose call,
since no implementation seems to need it.

R=dcarney@chromium.org
BUG=

Review URL: https://codereview.chromium.org/212893007

Patch from Daniel Vogelheim <vogelheim@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20326 ce2b1a6d-e550-0410-aec6-3dcde31c8c00