WebAudio: Remove legacy support for numerical values for Panner model and distance types.

Tests updated accordingly.

BUG=346522

Review URL: https://codereview.chromium.org/205343005

git-svn-id: svn://svn.chromium.org/blink/trunk@171076 bbb929c8-8fbe-4397-9dbb-9b2b20218538