Add Bogus initialization for external_elements_kind and fixed_elements_kind

R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/212553008

Patch from Haitao Feng <haitao.feng@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20315 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Don't pass the hole to SetElement when creating Array.observe change records

Also added comments to remind us why we were using the hole here in the first
place (it's used for the case where Object.observe, rather than Array.observe,
has been called on Array that's undergoing truncation).

BUG=356589
LOG=N
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/213823002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20316 ce2b1a6d-e550-0410-aec6-3dcde31c8c00