Provide default traits for PersistentValueMap

Re-try of issue 201643003. This caused linker errors on Win64, since the
linker insists on seeing the StrongMapTrait method implementations even
though they are never used. This will provide default implementations
for them.

R=dcarney@chromium.org

BUG=

Review URL: https://codereview.chromium.org/204343006

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20084 ce2b1a6d-e550-0410-aec6-3dcde31c8c00