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

core/fpdfapi/fpdf_page/cpdf_pageobjectlist.cpp

+// 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
+
+#include "core/fpdfapi/fpdf_page/cpdf_pageobjectlist.h"
+
+#include "core/fpdfapi/fpdf_page/pageint.h"
+#include "core/include/fpdfapi/fpdf_pageobj.h"
+#include "third_party/base/stl_util.h"
+
+CPDF_PageObject* CPDF_PageObjectList::GetPageObjectByIndex(int index) {
+  if (index < 0 || index >= pdfium::CollectionSize<int>(*this))
+    return nullptr;
+  return (*this)[index].get();
+}