Implement PersistentValueMap, a map that stores UniquePersistent values.
This is preparatory work to get rid of UnsafePersistent in blink.

The previous version had to be reverted due to timeouts in win32/Debug: https://codereview.chromium.org/197173002/

The timeouts happened because the STL version on that platform contains sanity checking code which opens a 'debug window' in the GUI, patiently waiting for the user to click ok/cancel/somethirdoption. It turns out, the cause for that debug window was totally valid and the test had a use-after-free issue.

The 1st patch set is the code as before. The 2nd patch set contains the fix.

Related blink changes are here: https://codereview.chromium.org/180363004/

This patch is largely based on https://codereview.chromium.org/175503003/, with some methods added to support the blink change mentioned above.

BUG=
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/197263002

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19873 ce2b1a6d-e550-0410-aec6-3dcde31c8c00