A little code refactoring on VirtualMemory::Commit for platform-win32

Review URL: https://codereview.chromium.org/14244006

Patch from Haitao Feng <haitao.feng@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14261 ce2b1a6d-e550-0410-aec6-3dcde31c8c00