Make the windows_version.h functions threadsafe by using a singleton.

base/win/windows_version.h

Index: base/win/windows_version.h
===================================================================
--- base/win/windows_version.h	(revision 79219)
+++ base/win/windows_version.h	(working copy)
@@ -12,15 +12,15 @@
 namespace win {
 
 // NOTE: Keep these in order so callers can do things like
-// "if (GetWinVersion() > WINVERSION_2000) ...".  It's OK to change the values,
-// though.
+// "if (base::win::GetVersion() > base::win::VERSION_2000) ...".  It's OK to
+// change the values, though.
 enum Version {
   VERSION_PRE_2000 = 0,     // Not supported
   VERSION_2000 = 1,         // Not supported
   VERSION_XP = 2,
   VERSION_SERVER_2003 = 3,  // Also includes Windows XP Professional x64 edition
   VERSION_VISTA = 4,
-  VERSION_2008 = 5,
+  VERSION_SERVER_2008 = 5,
   VERSION_WIN7 = 6,
 };