remove getcurrent from stubs

R=svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/23756009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16710 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
remove OS::StackWalk

R=bmeurer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/23703013

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16711 ce2b1a6d-e550-0410-aec6-3dcde31c8c00