Move libplatform files into v8::platform namespace

BUG=none
R=jkummerow@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/336313017

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22107 ce2b1a6d-e550-0410-aec6-3dcde31c8c00