Prevent crash due to XMLDocumentParser destruction

This patch prevents a crash in XMLDocumentParser::append due to the
parser being destructed through doWrite. Destructing the parser can
lead to the subsequent isStopped() check to fail to return, but by
keeping the parser alive we ensure isStopped() correctly exits.

BUG=278908

Review URL: https://chromiumcodereview.appspot.com/23456031

git-svn-id: svn://svn.chromium.org/blink/trunk@157914 bbb929c8-8fbe-4397-9dbb-9b2b20218538