Split off CPDF_Document into its own .cpp/.h files.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 007f2ae7d22bc1b1a0855374610e263dbe3262cd..06f33a72b76764076bb3e92a6da35d53e8f1f26d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -237,6 +237,7 @@ static_library("fpdfdoc") {
 
 static_library("fpdfapi") {
   sources = [
+    "core/include/fpdfapi/cpdf_document.h",
     "core/include/fpdfapi/fpdf_module.h",
     "core/include/fpdfapi/fpdf_objects.h",
     "core/include/fpdfapi/fpdf_page.h",
@@ -331,10 +332,10 @@ static_library("fpdfapi") {
     "core/src/fpdfapi/fpdf_page/fpdf_page_path.cpp",
     "core/src/fpdfapi/fpdf_page/fpdf_page_pattern.cpp",
     "core/src/fpdfapi/fpdf_page/pageint.h",
+    "core/src/fpdfapi/fpdf_parser/cpdf_document.cpp",
     "core/src/fpdfapi/fpdf_parser/cpdf_syntax_parser.cpp",
     "core/src/fpdfapi/fpdf_parser/cpdf_syntax_parser.h",
     "core/src/fpdfapi/fpdf_parser/fpdf_parser_decode.cpp",
-    "core/src/fpdfapi/fpdf_parser/fpdf_parser_document.cpp",
     "core/src/fpdfapi/fpdf_parser/fpdf_parser_encrypt.cpp",
     "core/src/fpdfapi/fpdf_parser/fpdf_parser_fdf.cpp",
     "core/src/fpdfapi/fpdf_parser/fpdf_parser_objects.cpp",