Build fix, destructor must be virtual

RuntimeRegistryObserver has virtual methods, so destructor must be virtual.
This check is enforced on the Mac and the build fails.
Don't use namespace directive in global context in header

On Mac, the build fails with
../../xwalk/sysapps/raw_socket/raw_socket_object.h:11:31: error: using namespace directive in global context in header [-Werror,-Wheader-hygiene]
Merge pull request #1124 from gramakri-intel/buildfix

Build fix, destructor must be virtual