Bug 1736248 - Update the charset source if non-ASCII is seen after the first detector guess but the encoding does not change. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D133731

parser/html/nsHtml5StreamParser.cpp

@@ -2273,6 +2273,11 @@ void nsHtml5StreamParser::ParseAvailableData() {
                     source == kCharsetFromFinalUserForcedAutoDetection);
                 mTreeBuilder->NeedsCharsetSwitchTo(encoding, source, 0);
                 requestedReload = true;
+              } else if (mCharsetSource ==
+                             kCharsetFromInitialAutoDetectionASCII &&
+                         mDetectorHasSeenNonAscii) {
+                mCharsetSource = source;
+                mTreeBuilder->SetDocumentCharset(mEncoding, mCharsetSource);
               }
             }