Throw errors when changing channelCount and channelCountMode for ScriptProcessors

The channelCount and channelCountMode for a ScriptProcessor node
cannot be changed after creation.  Thus, throw errors if the
the user tries to change channelCount to a different value or tries to
change channelCountMode to something other than 'explicit'.

Also fixes a bug that creation of a ScriptProcessor did not set
channelCount to the number of input channels and the channelCountMode
to 'explicit'.

See https://github.com/WebAudio/web-audio-api/issues/357

Tests added.
BUG=418224

Review URL: https://codereview.chromium.org/605403002

git-svn-id: svn://svn.chromium.org/blink/trunk@182870 bbb929c8-8fbe-4397-9dbb-9b2b20218538