Getting rid of MS C++ warning when compiling GPR on Windows.

1>c:\users\jtattermusch\grpc_tar\src\core\support\sync_win32.c(97): warning C4244: 'function' : conversion from 'gpr_int64' to 'DWORD', possible loss of data
	Change on 2014/12/09 by jtattermusch <jtattermusch@google.com>
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=81724253