Add unit test for CFX_SAXReader.

Make the pending re-write CL smaller, and back-fill some test.
Virtualize the handler interface so that it can be mocked.

Change-Id: Icbbc6b6789ba135b73c6fd1bbb90f5aae6ab7649
Reviewed-on: https://pdfium-review.googlesource.com/4250
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 77bdeb5..9b879f5 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1878,12 +1878,14 @@
   deps = [
     ":pdfium",
     ":test_support",
+    "//testing/gmock",
     "//testing/gtest",
     "//testing/gtest:gtest_main",
   ]
   include_dirs = []
   if (pdf_enable_xfa) {
     sources += [
+      "core/fxcrt/xml/cfx_saxreader_unittest.cpp",
       "fxbarcode/pdf417/BC_PDF417HighLevelEncoder_unittest.cpp",
       "xfa/fde/cfde_txtedtbuf_unittest.cpp",
       "xfa/fde/css/cfde_cssdeclaration_unittest.cpp",