Make sure getAttribute() / setAttribute() callers use AtomicStrings

Source/core/html/HTMLDocument.cpp

Index: Source/core/html/HTMLDocument.cpp
diff --git a/Source/core/html/HTMLDocument.cpp b/Source/core/html/HTMLDocument.cpp
index ee9cb2434940a2bb51a4d59818ae24110c29277d..761a6e3093af1b111169f56524e2317c575d58b2 100644
--- a/Source/core/html/HTMLDocument.cpp
+++ b/Source/core/html/HTMLDocument.cpp
@@ -79,15 +79,15 @@ HTMLDocument::~HTMLDocument()
 {
 }
 
-String HTMLDocument::dir()
+const AtomicString& HTMLDocument::dir()
 {
     HTMLElement* b = body();
     if (!b)
-        return String();
+        return nullAtom;
     return b->getAttribute(dirAttr);
 }
 
-void HTMLDocument::setDir(const String& value)
+void HTMLDocument::setDir(const AtomicString& value)
 {
     HTMLElement* b = body();
     if (b)