Remove _FXBSTR* and calls to FX_BSTRC.

This CL removes _FXBSTR and changes FPDF_AbbrPair to hold
two FX_CHAR pointers. The two remaining uses of FX_BSTRC()
were in unused FDE_CSS defines which have been removed.

BUG=pdfium:151

Review URL: https://codereview.chromium.org/1847333004
diff --git a/BUILD.gn b/BUILD.gn
index b5d8950..855206e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1578,6 +1578,7 @@
     "core/fpdfapi/fpdf_font/fpdf_font_cid_unittest.cpp",
     "core/fpdfapi/fpdf_font/fpdf_font_unittest.cpp",
     "core/fpdfapi/fpdf_page/fpdf_page_parser_old_unittest.cpp",
+    "core/fpdfapi/fpdf_page/fpdf_page_parser_unittest.cpp",
     "core/fpdfapi/fpdf_parser/cpdf_object_unittest.cpp",
     "core/fpdfapi/fpdf_parser/cpdf_parser_unittest.cpp",
     "core/fpdfapi/fpdf_parser/cpdf_simple_parser_unittest.cpp",