Convert `obj` ToObject in Object.keys()

BUG=v8:3587
LOG=Y
R=arv@chromium.org, dslomov@chromium.org

Review URL: https://codereview.chromium.org/585373003

Patch from Caitlin Potter <caitpotter88@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24260 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Move string-related runtime functions into separate files.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/604703004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24261 ce2b1a6d-e550-0410-aec6-3dcde31c8c00