XMLHttpRequest should clear the response encoding if it clears responseText.

XMLHttpRequest did not clear the response encoding when it have should, for example, when abort() is called.
This caused character corruption in responseText if we re-opened the XHR object.

BUG=261140
R=abarth

Review URL: https://chromiumcodereview.appspot.com/20625004

git-svn-id: svn://svn.chromium.org/blink/trunk@155008 bbb929c8-8fbe-4397-9dbb-9b2b20218538