HTMLOptionElement.prototype.selected setter should not make an option dirty if it doesn't change it…

third_party/WebKit/Source/core/html/HTMLOptionElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLOptionElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLOptionElement.cpp b/third_party/WebKit/Source/core/html/HTMLOptionElement.cpp
index 07ba7a7af1bbffe62c550678cbb6e14b3b149caa..c95d5883273a61184f6a0442bef61f1ab6af46ea 100644
--- a/third_party/WebKit/Source/core/html/HTMLOptionElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLOptionElement.cpp
@@ -267,7 +267,17 @@ bool HTMLOptionElement::selectedForBinding() const
 
 void HTMLOptionElement::setSelectedForBinding(bool selected)
 {
+    bool wasSelected = m_isSelected;
     setSelected(selected);
+
+    // As of December 2015, the HTML specification says the dirtiness becomes

[tfarina, 2015/12/23 23:04:36]

is it worth pointing to the link for that specification?

+    // true by |selected| setter unconditionally. However it caused a real bug,
+    // crbug.com/570367, and is not compatible with other browsers.
+    // Firefox seems not to set dirtiness if an option is owned by a select
+    // element and selectedness is not changed.
+    if (ownerSelectElement() && wasSelected == m_isSelected)
+        return;
+
     m_isDirty = true;
 }