WebAudio: Simplify ScriptProcessorNode by CrossThreadTask.

CrossThreadTask automtically protects a passed pointer. We can remove
m_keepAliveWhileEventDispatching.

BUG=392788

Review URL: https://codereview.chromium.org/396893003

git-svn-id: svn://svn.chromium.org/blink/trunk@178254 bbb929c8-8fbe-4397-9dbb-9b2b20218538