Document attribute charset and its aliases to return UTF-8

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index d305ffb3d2a566c0ec7c61d0db9b06639882d94a..8f9db56cc4018a549df0efaa59564a7e9c546995 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -4218,6 +4218,7 @@ void Document::setEncodingData(const DocumentEncodingData& newData)
         m_titleElement->setTextContent(correctlyDecodedTitle);
     }
 
+    ASSERT(newData.encoding().isValid());
     m_encodingData = newData;
 
     // FIXME: Should be removed as part of https://code.google.com/p/chromium/issues/detail?id=319643