Revert build of pdfium_unittests.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index a442e79394685824f1ae5ec59c4e8b4d3cb20210..2076afb87aa2323ef2c15402e1fbe642c3ff3935 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -791,16 +791,3 @@ static_library("formfiller") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ ":pdfium_config", "//build/config/compiler:no_chromium_code" ]
 }
-
-executable("pdfium_unittests") {
-  testonly = true
-  sources = [
-    "core/src/fxcrt/fx_basic_bstring_unittest.cpp",
-  ]
-  deps = [
-    ":pdfium",
-    "//base",
-    "//base/test:run_all_unittests",
-    "//testing/gtest",
-  ]
-}