Use parseFromString type arg to set mime type.

third_party/WebKit/Source/core/xml/DOMParser.cpp

 /*
  *  Copyright (C) 2003, 2006, 2008 Apple Inc. All rights reserved.
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public
  *  License as published by the Free Software Foundation; either
  *  version 2 of the License, or (at your option) any later version.
  *
  *  This library is distributed in the hope that it will be useful,
  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 11 matching lines @@
 #include "core/dom/DOMImplementation.h"
 #include "platform/weborigin/SecurityOrigin.h"
 #include "wtf/text/WTFString.h"
 
 namespace blink {
 
 Document* DOMParser::parseFromString(const String& str, const String& type) {
   Document* doc = DOMImplementation::createDocument(
       type, DocumentInit(KURL(), nullptr, m_contextDocument), false);
   doc->setContent(str);
+  doc->setMimeType(AtomicString(type));
   if (m_contextDocument) {
     doc->setURL(m_contextDocument->url());
     doc->setSecurityOrigin(m_contextDocument->getSecurityOrigin());
   }
   return doc;
 }
 
 DOMParser::DOMParser(Document& document)
     : m_contextDocument(document.contextDocument()) {}
 
 DEFINE_TRACE(DOMParser) {
   visitor->trace(m_contextDocument);
 }
 
 }  // namespace blink