Move TextResourceDecoder from loader/ to fetch/

TextResourceDecoder is used by many classes including one from fetch/
and it is not tied to page navigation which loader/ is responsible for.

BUG=280939
R=japhet,abarth
TEST=none

Review URL: https://chromiumcodereview.appspot.com/23623012

git-svn-id: svn://svn.chromium.org/blink/trunk@156978 bbb929c8-8fbe-4397-9dbb-9b2b20218538