WebAudio: Add factory functions to AudioNodeInput and AudioNodeOutput.

Also, AudioNode::addInput is used with a fixed argument. This CL fold the
argument into addInput implementation.

BUG=
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/356833004

git-svn-id: svn://svn.chromium.org/blink/trunk@176975 bbb929c8-8fbe-4397-9dbb-9b2b20218538