Added -Woverloaded-virtual to GCC flags, syncing scons and gyp builds.

Review URL: https://chromiumcodereview.appspot.com/9430043

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10790 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

SConstruct

@@ -296,6 +296,7 @@ V8_EXTRA_FLAGS = {
                        '-Werror',
                        '-W',
                        '-Wno-unused-parameter',
+                       '-Woverloaded-virtual',
                        '-Wnon-virtual-dtor']
     },
     'os:win32': {