Mark AtomicString(StringImpl*) constructor as explicit

Source/core/editing/SetNodeAttributeCommand.cpp

Index: Source/core/editing/SetNodeAttributeCommand.cpp
diff --git a/Source/core/editing/SetNodeAttributeCommand.cpp b/Source/core/editing/SetNodeAttributeCommand.cpp
index b4bd12f3a72084e66f031856a30c09751abe85f4..2d4b7ee8f9dd045b33bbd896bdc5b1b2893c87a8 100644
--- a/Source/core/editing/SetNodeAttributeCommand.cpp
+++ b/Source/core/editing/SetNodeAttributeCommand.cpp
@@ -50,8 +50,7 @@ void SetNodeAttributeCommand::doApply()
 void SetNodeAttributeCommand::doUnapply()
 {
     m_element->setAttribute(m_attribute, m_oldValue);
-    StringImpl* nullString = 0;
-    m_oldValue = nullString;
+    m_oldValue = nullAtom;
 }
 
 } // namespace WebCore