Add CPDF_Document::GetPage() unittests

BUILD.gn

 # Copyright 2016 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.
 
 import("//build_overrides/v8.gni")
 import("//testing/test.gni")
 import("pdfium.gni")
 
 config("pdfium_common_config") {
   cflags = []
@@ skipping 1616 matching lines @@
   }
 }
 
 test("pdfium_unittests") {
   sources = [
     "core/fpdfapi/font/fpdf_font_cid_unittest.cpp",
     "core/fpdfapi/font/fpdf_font_unittest.cpp",
     "core/fpdfapi/page/fpdf_page_parser_old_unittest.cpp",
     "core/fpdfapi/page/fpdf_page_parser_unittest.cpp",
     "core/fpdfapi/parser/cpdf_array_unittest.cpp",
+    "core/fpdfapi/parser/cpdf_document_unittest.cpp",
     "core/fpdfapi/parser/cpdf_object_unittest.cpp",
     "core/fpdfapi/parser/cpdf_parser_unittest.cpp",
     "core/fpdfapi/parser/cpdf_simple_parser_unittest.cpp",
     "core/fpdfapi/parser/cpdf_syntax_parser_unittest.cpp",
     "core/fpdfapi/parser/fpdf_parser_decode_unittest.cpp",
     "core/fpdfdoc/cpdf_filespec_unittest.cpp",
     "core/fpdfdoc/cpdf_formfield_unittest.cpp",
     "core/fpdftext/fpdf_text_int_unittest.cpp",
     "core/fxcodec/codec/fx_codec_jpx_unittest.cpp",
     "core/fxcodec/jbig2/JBig2_Image_unittest.cpp",
@@ skipping 102 matching lines @@
 }
 
 if (pdf_is_standalone) {
   source_set("samples") {
     testonly = true
     deps = [
       "//samples",
     ]
   }
 }