XMLHttpRequest.responseXML.characterSet may be inaccurate

https://bugs.webkit.org/show_bug.cgi?id=258148

Reviewed by Ryosuke Niwa.

When constructing a document in XMLHttpRequest::responseXML(), we would fail to pass
the TextDecoder we used on the new document (unlike what we do in the HTML parser).
This means that Document::characterSet() would not be able to determine the encoding
our decoder used and would always return UTF-8.

* LayoutTests/imported/w3c/web-platform-tests/html/syntax/charset/xhr-expected.txt:
* Source/WebCore/xml/XMLHttpRequest.cpp:

Canonical link: https://commits.webkit.org/265210@main

LayoutTests/imported/w3c/web-platform-tests/html/syntax/charset/xhr-expected.txt

@@ -1,24 +1,24 @@
 
-FAIL Check after-1kb.html assert_equals: Check characterSet expected "windows-1251" but got "UTF-8"
+PASS Check after-1kb.html
 PASS Check after-bogus-after-1kb.html
-FAIL Check after-bogus.html assert_equals: Check characterSet expected "windows-1251" but got "UTF-8"
+PASS Check after-bogus.html
 PASS Check after-head-after-1kb-crlf.html
 PASS Check after-head-after-1kb.html
-FAIL Check after-head-in-1kb-crlf.html assert_equals: Check characterSet expected "windows-1251" but got "UTF-8"
-FAIL Check after-head-in-1kb.html assert_equals: Check characterSet expected "windows-1251" but got "UTF-8"
-FAIL Check baseline.html assert_equals: Check characterSet expected "windows-1251" but got "UTF-8"
+PASS Check after-head-in-1kb-crlf.html
+PASS Check after-head-in-1kb.html
+PASS Check baseline.html
 PASS Check document-write.html
 PASS Check in-comment.html
 PASS Check in-noscript-after-template-after-1kb.html
-FAIL Check in-object.html assert_equals: Check characterSet expected "windows-1251" but got "UTF-8"
+PASS Check in-object.html
 PASS Check in-script.html
 PASS Check in-style.html
-FAIL Check in-svg.html assert_equals: Check characterSet expected "windows-1251" but got "UTF-8"
+PASS Check in-svg.html
 PASS Check in-svg-in-cdata.html
 PASS Check in-template-after-1kb.html
-FAIL Check in-template.html assert_equals: Check characterSet expected "windows-1251" but got "UTF-8"
+PASS Check in-template.html
 PASS Check in-title.html
-FAIL Check ncr.html assert_equals: Check characterSet expected "windows-1251" but got "UTF-8"
-FAIL Check non-ascii-in-comment-before.html assert_equals: Check characterSet expected "windows-1251" but got "UTF-8"
-FAIL Check non-ascii-in-title-before.html assert_equals: Check characterSet expected "windows-1251" but got "UTF-8"
+PASS Check ncr.html
+PASS Check non-ascii-in-comment-before.html
+PASS Check non-ascii-in-title-before.html
 

Source/WebCore/xml/XMLHttpRequest.cpp

@@ -190,6 +190,8 @@ ExceptionOr<Document*> XMLHttpRequest::responseXML()
             responseDocument->setSecurityOriginPolicy(context.securityOriginPolicy());
             responseDocument->overrideMIMEType(mimeType);
             responseDocument->setContent(m_responseBuilder.toStringPreserveCapacity());
+            if (m_decoder)
+                responseDocument->setDecoder(m_decoder.copyRef());
 
             if (!responseDocument->wellFormed())
                 m_responseDocument = nullptr;