WebAudio: Make the arguments of AudioParam::connect and disconnect references.

We can remove unnecessary null checks safely.

BUG=

Review URL: https://codereview.chromium.org/393023007

git-svn-id: svn://svn.chromium.org/blink/trunk@178266 bbb929c8-8fbe-4397-9dbb-9b2b20218538