Consistently start (almost) all AstPrinter::VisitFoo functions with IndentedScope.

This makes IndentedScope the single point where we can print additional info in
the future, like e.g. AST IDs (at least when the TODOs are fixed). Minor other
cleanups.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/15060005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14592 ce2b1a6d-e550-0410-aec6-3dcde31c8c00