Removed refcounting from TextResourceDecoder

It's no longer needed anywhere, and this makes the transition to performing decoding on the parser thread more robust.

Builds on https://codereview.chromium.org/73293003/

R=abarth@chromium.org
BUG=277886

Review URL: https://codereview.chromium.org/74493002

git-svn-id: svn://svn.chromium.org/blink/trunk@162316 bbb929c8-8fbe-4397-9dbb-9b2b20218538