Componentization: make core classes visible for blink_tests and blink_web.

Source/core/dom/Attr.h

Index: Source/core/dom/Attr.h
diff --git a/Source/core/dom/Attr.h b/Source/core/dom/Attr.h
index b46245063356bf7194f001c14dae4b9aa4c9709a..18d44adb48a41bb22cc510afbf3d5dcc42241b04 100644
--- a/Source/core/dom/Attr.h
+++ b/Source/core/dom/Attr.h
@@ -25,12 +25,13 @@
 #ifndef Attr_h
 #define Attr_h
 
+#include "core/CoreExport.h"
 #include "core/dom/Node.h"
 #include "core/dom/QualifiedName.h"
 
 namespace blink {
 
-class Attr final : public Node {
+class CORE_EXPORT Attr final : public Node {
     DEFINE_WRAPPERTYPEINFO();
 public:
     static PassRefPtrWillBeRawPtr<Attr> create(Element&, const QualifiedName&);