OfflineAudioContext should support up to 32 channels.

Previously, only 10 channels were supported for an
OfflineAudioContext. Although the WebAudio specification doesn't explicitly say
what the limit is, generally at least 32 channels are required to be
supported for all nodes.

Added a test that creating an offline context with 32 channels works.

BUG=362133

Review URL: https://codereview.chromium.org/232843004

git-svn-id: svn://svn.chromium.org/blink/trunk@171480 bbb929c8-8fbe-4397-9dbb-9b2b20218538