Get rid of custom bindings for AudioContext's constructor

Get rid of custom bindings for AudioContext's constructor by leveraging
existing generator functionality and adding a new factory method to
AudioContext that constructs an OfflineAudioContext.

R=haraken, kbr@chromium.org
TEST=webaudio/audionode.html
BUG=

Review URL: https://codereview.chromium.org/27037002

git-svn-id: svn://svn.chromium.org/blink/trunk@159613 bbb929c8-8fbe-4397-9dbb-9b2b20218538