Sync OS::GetCurrentThreadId with chromium's/blink's implementation

Compare third_party/WebKit/Source/wtf/ThreadingPthreads.cpp

BUG=v8:3620
R=svenpanne@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/649553002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24511 ce2b1a6d-e550-0410-aec6-3dcde31c8c00