Add NativeWeakMap to v8.h

Add NativeWeakMap to v8.h

A new map wich references its keys weakly is added to v8.h. Internally it uses the same storage as JSWeakMap but doesn't depend on the JavaScript part of WeakMap implementation in weak-collection.js, hence it can be instantiated without entering any context.

BUG=chromium:437416
LOG=Y

Committed: https://crrev.com/a559367956d870de38375a51e919a2c06c004e26
Cr-Commit-Position: refs/heads/master@{#26451}

[yurys, 2015-02-05 08:32:41]

yurys@chromium.org changed reviewers:
+ dcarney@chromium.org, jochen@chromium.org, mstarzinger@chromium.org,
rossberg@chromium.org

[yurys, 2015-02-05 08:32:41]

This patch is a reworked version of r26425. It was modified in accord with
dcarney's comments. Instead of embedding UniquePersistent field NativeWeakMap
has been turned into a heap object. This should vanish MSVS warning.

Also applied mstarzinger's r26428.

[dcarney, 2015-02-05 08:34:57]

lgtm wrt to v8.h and api.*

[Michael Starzinger, 2015-02-05 08:57:39]

LGTM (rubber-stamped).

[yurys, 2015-02-05 09:19:20]

The CQ bit was checked by yurys@chromium.org

[commit-bot: I haz the power, 2015-02-05 09:19:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/900123003/20001

[commit-bot: I haz the power, 2015-02-05 09:40:25]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-05 09:40:38]

Patchset 2 (id:??) landed as
https://crrev.com/a559367956d870de38375a51e919a2c06c004e26
Cr-Commit-Position: refs/heads/master@{#26451}