XHR: Abort DocumentParser on internalAbort

Before this CL, XHR didn't abort |m_responseDocumentParser| when the
request was cancelled.  This CL adds a call to |DocumentParser::stopParsing()|
to force cancel parsing when the request is aborted.

This CL also fixes |m_responseDocumentParser| destruction by moving the pointer
clear to |clearVariablesForLoading| and calling |detach()| on non-oilpan builds.

BUG=409461

Review URL: https://codereview.chromium.org/521403002

git-svn-id: svn://svn.chromium.org/blink/trunk@181247 bbb929c8-8fbe-4397-9dbb-9b2b20218538