Throw DOMException when invoked Document::execCommand on non-{X,}HTML documents

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 42bcc89466e3fed4f2b67f9947c25ad6623ec05b..8ab8ad07e2f97f2033d1b5add2bc0d162b1e4b7f 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -4249,6 +4249,9 @@ static Editor::Command command(Document* document, const String& commandName)
 
 bool Document::execCommand(const String& commandName, bool, const String& value)
 {
+    if (!isHTMLDocument() && !isXHTMLDocument())
+        return false;
+
     // We don't allow recursive |execCommand()| to protect against attack code.
     // Recursive call of |execCommand()| could be happened by moving iframe
     // with script triggered by insertion, e.g. <iframe src="javascript:...">