Replace OS::NumberOfCores() with CPU::NumberOfProcessorsOnline().

Replace OS::NumberOfCores() with CPU::NumberOfProcessorsOnline().

The name NumberOfCores is misleading, as it does not return the
actual number of cores. While NumberOfProcessorsOnline is also
not a great name, it's at least consistent with the operating
system terminology.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/23655004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16392 ce2b1a6d-e550-0410-aec6-3dcde31c8c00