Add "own" symbols support.

"Own" symbols are symbols that can only denote own properties of
objects.

R=hpayer@chromium.org, verwaest@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=23056

Review URL: https://codereview.chromium.org/464473002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23083 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Old space cannot be assumed to be iterable between GCs, even if swept precisely.

This is because allocation folding can cause uninitialized and hence uniterable fragments. Trying to inspect them causes SizeFromMap() crashes.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/467693002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23084 ce2b1a6d-e550-0410-aec6-3dcde31c8c00