In the process, make ElementsAccessors work internally more seamlessly with FixedArrayBase.

R=svenpanne@chromium.org
BUG=none
TEST=none

Review URL: http://codereview.chromium.org/7618012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8930 ce2b1a6d-e550-0410-aec6-3dcde31c8c00