Simplify PDFium by removing code that's not used in the open source repo.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 2e471637d49a3f67c727aebf1fefd0435d648edb..9b8fdf7cba01a17194bdf7232aa5303c401f8f1a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -7,7 +7,6 @@ pdf_use_skia = false
 config("pdfium_config") {
   cflags = []
   defines = [
-    "FOXIT_CHROME_BUILD",
     "_FXFT_VERSION_=2501",
     "_FPDFSDK_LIB",
     "_NO_GDIPLUS_",  # workaround text rendering issues on Windows
@@ -43,7 +42,6 @@ static_library("pdfium") {
     "fpdfsdk/include/fpdfdoc.h",
     "fpdfsdk/include/fpdfedit.h",
     "fpdfsdk/include/fpdfformfill.h",
-    "fpdfsdk/include/fpdfoom.h",
     "fpdfsdk/include/fpdftext.h",
     "fpdfsdk/include/fpdfview.h",
     "fpdfsdk/include/fpdf_dataavail.h",
@@ -61,7 +59,6 @@ static_library("pdfium") {
     "fpdfsdk/src/fpdfeditimg.cpp",
     "fpdfsdk/src/fpdfeditpage.cpp",
     "fpdfsdk/src/fpdfformfill.cpp",
-    "fpdfsdk/src/fpdfoom.cpp",
     "fpdfsdk/src/fpdfppo.cpp",
     "fpdfsdk/src/fpdfsave.cpp",
     "fpdfsdk/src/fpdftext.cpp",
@@ -282,7 +279,6 @@ static_library("fpdfapi") {
     "core/src/fpdfapi/fpdf_page/fpdf_page_graph_state.cpp",
     "core/src/fpdfapi/fpdf_page/fpdf_page_image.cpp",
     "core/src/fpdfapi/fpdf_page/fpdf_page_parser.cpp",
-    "core/src/fpdfapi/fpdf_page/fpdf_page_parser_new.cpp",
     "core/src/fpdfapi/fpdf_page/fpdf_page_parser_old.cpp",
     "core/src/fpdfapi/fpdf_page/fpdf_page_path.cpp",
     "core/src/fpdfapi/fpdf_page/fpdf_page_pattern.cpp",
@@ -634,7 +630,6 @@ static_library("fxge") {
     "core/src/fxge/ge/fx_ge_device.cpp",
     "core/src/fxge/ge/fx_ge_font.cpp",
     "core/src/fxge/ge/fx_ge_fontmap.cpp",
-    "core/src/fxge/ge/fx_ge_linux.cpp",
     "core/src/fxge/ge/fx_ge_path.cpp",
     "core/src/fxge/ge/fx_ge_ps.cpp",
     "core/src/fxge/ge/fx_ge_text.cpp",