Provide default traits for PersistentValueMap

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=

Committed: https://code.google.com/p/v8/source/detail?r=20084

[vogelheim, 2014-03-19 15:18:29]

Re-trying https://codereview.chromium.org/201643003


So... I tried several variants to fix this, but without any real success. This
version - which provides empty default implementations - seems to work fine. At
least it does on the trybots.

I tried various versions of inline & V8_INLINE, but didn't get any of them to
compile (on Linux) without actually providing inline methods. That is, inline
won't help us avoid the unused default impls.

Let's briefly discuss what to do; but this is currently my best guess.

[dcarney, 2014-03-19 15:23:13]

lgtm

[dcarney, 2014-03-19 15:35:08]

Committed patchset #2 manually as r20084 (presubmit successful).