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

Review URL: https://codereview.chromium.org/1148383007

Cr-Commit-Position: refs/heads/master@{#28640}