Remove support for prefixed WebAudio

Support for webkitAudioContext and webkitOfflineAudioContext is
removed. Also, the basic legacy methods in AudioContext.idl are
removed as well. This brings Blink's WebAudio support partially in
line with the W3C spec. There are some legacy methods and attributes
that still need to be removed, but that will be done in followup CLs.

Legacy methods removed:
noteOn
noteGrainOn
noteOff
creatGainNode
createDelayNode
setTargetValueAtTime
3-arg AudioContext constructor that created an OfflineAudioContext

Some layout tests are also updated to support this change.

This is a follow up to issue 302751 which added support for the unprefixed WebAudio, but still supported the legacy prefixed WebAudio.

BUG=358398

Review URL: https://codereview.chromium.org/213503002

git-svn-id: svn://svn.chromium.org/blink/trunk@170985 bbb929c8-8fbe-4397-9dbb-9b2b20218538