Merge to XFA: Fix subtle issues in opj_skip_from_memory and add unit tests.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 830e7d1309a685646c57cf62d3e45022fe893c42..ab230b44d8228adf32194a7807b548f7c50579b4 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1554,6 +1554,7 @@ static_library("xfa") {
 test("pdfium_unittests") {
   sources = [
     "core/src/fpdfapi/fpdf_parser/fpdf_parser_decode_unittest.cpp",
+    "core/src/fxcodec/codec/fx_codec_jpx_unittest.cpp",
     "core/src/fxcrt/fx_basic_bstring_unittest.cpp",
     "core/src/fxcrt/fx_basic_wstring_unittest.cpp",
     "testing/fx_string_testhelpers.cpp",