Implement PersistentValueMap, a map that stores UniquePersistent values. Part 2.

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

Committed: https://code.google.com/p/v8/source/detail?r=19873

[vogelheim, 2014-03-12 16:29:40]

PTAL.

2nd try for PersistentValueMap, w/ fix for user-after-free discovered by
Win32/debug bot.

[dcarney, 2014-03-13 07:07:52]

lgtm, landing

[dcarney, 2014-03-13 07:11:07]

Committed patchset #2 manually as r19873 (presubmit successful).