Add {Map,Set}::AsArray to the API

Add {Map,Set}::AsArray to the API

These return arrays representing the current contents of the given
Map/Set. They are similar to what would be returned by the JS code:

  Array.from(collection)

except that they are guaranteed side-effect free.

This will be useful in implementing structured clone which, as
specified in HTML, speaks in terms of the internal [[MapData]]
and [[SetData]] slots without going through the exposed iteration
ES semantics.

BUG=v8:3340
LOG=y

Committed: https://crrev.com/a8d9c58b1f618f518dc3a5a73023e6847544979f
Cr-Commit-Position: refs/heads/master@{#28640}

[jochen (gone - plz use gerrit), 2015-05-26 15:43:02]

jochen@chromium.org changed reviewers:
+ jochen@chromium.org

[jochen (gone - plz use gerrit), 2015-05-26 15:43:02]

lgtm

[arv (Not doing code reviews), 2015-05-26 16:33:22]

arv@chromium.org changed reviewers:
+ arv@chromium.org

[arv (Not doing code reviews), 2015-05-26 16:33:22]

LGTM

[adamk, 2015-05-26 17:44:47]

The CQ bit was checked by adamk@chromium.org

[adamk, 2015-05-26 17:44:47]

The patchset sent to the CQ was uploaded after l-g-t-m from arv@chromium.org,
jochen@chromium.org
Link to the patchset: https://codereview.chromium.org/1148383007/#ps20001
(title: "Rebased")

[commit-bot: I haz the power, 2015-05-26 17:45:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1148383007/20001

[adamk, 2015-05-26 17:45:20]

The CQ bit was unchecked by adamk@chromium.org

[adamk, 2015-05-26 17:45:33]

The CQ bit was checked by adamk@chromium.org

[commit-bot: I haz the power, 2015-05-26 17:46:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1148383007/20001

[commit-bot: I haz the power, 2015-05-26 18:13:27]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-05-26 18:13:38]

Patchset 2 (id:??) landed as
https://crrev.com/a8d9c58b1f618f518dc3a5a73023e6847544979f
Cr-Commit-Position: refs/heads/master@{#28640}