Throw exceptions for invalid number of channels for ConvolverNode

The ConvolverNode only supports 1, 2 or 4 channels and must throw an
exception for any other number of channels.

BUG=563379
TEST=convolver-channels.html

Review URL: https://codereview.chromium.org/1497803002

Cr-Commit-Position: refs/heads/master@{#363024}
3 files changed