Introduce AXObjectCacheBase in core/ and remove WebNode dependency

third_party/WebKit/Source/core/dom/AXObjectCacheBase.cpp

Index: third_party/WebKit/Source/core/dom/AXObjectCacheBase.cpp
diff --git a/third_party/WebKit/Source/core/dom/AXObjectCacheBase.cpp b/third_party/WebKit/Source/core/dom/AXObjectCacheBase.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..3f1c1bd84ac93049a30fd1b0d2f5a8448cddf89f
--- /dev/null
+++ b/third_party/WebKit/Source/core/dom/AXObjectCacheBase.cpp
@@ -0,0 +1,16 @@
+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/dom/AXObjectCacheBase.h"
+
+#include "core/CoreExport.h"
+#include "core/dom/AXObjectCache.h"
+
+namespace blink {
+
+AXObjectCacheBase::~AXObjectCacheBase() {}
+
+AXObjectCacheBase::AXObjectCacheBase() {}
+
+}  // namespace blink