Move ReverbConvolver to the new threading API

also delete some dead code.

BUG=301514
R=abarth@chromium.org, kbr@chromium.org, rtoy@google.com

Review URL: https://codereview.chromium.org/66303003

git-svn-id: svn://svn.chromium.org/blink/trunk@161728 bbb929c8-8fbe-4397-9dbb-9b2b20218538