bindings: make Nullable::get() return const reference
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@178902 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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