Move declaration of SerializedScopeInfo from variables.h to objects.h

Move declaration of SerializedScopeInfo from variables.h to objects.h

This eliminates compile-errors when assigning Handle<SerializedScopeInfo> to
Handle<Object> in a place where the declaration was not available because
variables.h was not included.

As a result I had to also move the enum Variable::Mode to v8globals.h and
rename it to VariableMode.
Review URL: http://codereview.chromium.org/8221004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9575 ce2b1a6d-e550-0410-aec6-3dcde31c8c00