Introduce AXObjectCacheBase in core/ and remove WebNode dependency

third_party/WebKit/Source/modules/accessibility/AXObjectCacheImpl.h

Index: third_party/WebKit/Source/modules/accessibility/AXObjectCacheImpl.h
diff --git a/third_party/WebKit/Source/modules/accessibility/AXObjectCacheImpl.h b/third_party/WebKit/Source/modules/accessibility/AXObjectCacheImpl.h
index c01b3fb41058cd91313960b6d06cf0de943d4c6d..4772d6d54c9dbb83fc5344c31eff2c0d6884fef1 100644
--- a/third_party/WebKit/Source/modules/accessibility/AXObjectCacheImpl.h
+++ b/third_party/WebKit/Source/modules/accessibility/AXObjectCacheImpl.h
@@ -30,7 +30,7 @@
 #define AXObjectCacheImpl_h
 
 #include <memory>
-#include "core/dom/AXObjectCache.h"
+#include "core/dom/AXObjectCacheBase.h"
 #include "modules/ModulesExport.h"
 #include "modules/accessibility/AXObject.h"
 #include "platform/wtf/Forward.h"
@@ -44,7 +44,7 @@ class HTMLAreaElement;
 class FrameView;
 
 // This class should only be used from inside the accessibility directory.
-class MODULES_EXPORT AXObjectCacheImpl : public AXObjectCache {
+class MODULES_EXPORT AXObjectCacheImpl : public AXObjectCacheBase {
   WTF_MAKE_NONCOPYABLE(AXObjectCacheImpl);
 
  public:
@@ -128,7 +128,7 @@ class MODULES_EXPORT AXObjectCacheImpl : public AXObjectCache {
   AXObject* GetOrCreate(AbstractInlineTextBox*);
 
   // will only return the AXObject if it already exists
-  AXObject* Get(Node*);
+  AXObject* Get(Node*) override;
   AXObject* Get(LayoutObject*);
   AXObject* Get(AbstractInlineTextBox*);