Create pdfium_unittests binary.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 2076afb87aa2323ef2c15402e1fbe642c3ff3935..a442e79394685824f1ae5ec59c4e8b4d3cb20210 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -791,3 +791,16 @@ 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",
+  ]
+}