WebAudio: Use references instead of pointers in AudioNodeInput and AudioNodeOutput.

We can remove some unnecessary assertions and null checks.

BUG=392788
TEST=none; No behavior changes

Review URL: https://codereview.chromium.org/386343004

git-svn-id: svn://svn.chromium.org/blink/trunk@178033 bbb929c8-8fbe-4397-9dbb-9b2b20218538