Warning: this change will require Chrome changes. See
https://codereview.chromium.org/803173009/#msg6.
Added ProxyBindings and StubBindings methods that expose
bindings-related properties for generated Proxy and Stub classes
without risking name collisions.
Outgoing "in/out" interface valued parameters are now specified as
functions that are applied to the out value.
Note: the leading capital in the Proxy,StubBindings() methods is
unconventional. It seemed OK given their unusual role: they can be
viewed as transformers from "Foo" objects to "FooBindings" objects.
BUG=
R=esprehn@chromium.org
Review URL: https://codereview.chromium.org/803173009