RTCPeerConnection: remove redundant throwing for callback args.

The generated binding code checks that the callback arguments are
callable, reporting the required TypeError. Hence, implementation
methods can simply assume that the supplied callback argument is
valid.

R=
BUG=

Review URL: https://codereview.chromium.org/177873002

git-svn-id: svn://svn.chromium.org/blink/trunk@167692 bbb929c8-8fbe-4397-9dbb-9b2b20218538