bindings: make Nullable::get() return const reference

The Nullable template can contain a large value. It would be better
not to create a copy for each get() call.

BUG=None

Review URL: https://codereview.chromium.org/413413002

git-svn-id: svn://svn.chromium.org/blink/trunk@178902 bbb929c8-8fbe-4397-9dbb-9b2b20218538