Move core/include/fpdfapi/fpdf_page.h to correct locations.

core/fpdfapi/fpdf_page/cpdf_pageobjectlist.h

+// Copyright 2016 PDFium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
+
+#ifndef CORE_FPDFAPI_FPDF_PAGE_CPDF_PAGEOBJECTLIST_H_
+#define CORE_FPDFAPI_FPDF_PAGE_CPDF_PAGEOBJECTLIST_H_
+
+#include <deque>
+#include <memory>
+
+class CPDF_PageObject;
+
+class CPDF_PageObjectList
+    : public std::deque<std::unique_ptr<CPDF_PageObject>> {
+ public:
+  CPDF_PageObject* GetPageObjectByIndex(int index);
+};
+
+#endif  // CORE_FPDFAPI_FPDF_PAGE_CPDF_PAGEOBJECTLIST_H_