Add support for extensions_unittests to the GN build

third_party/protobuf/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 if (is_win) {
   config_h_dir = "vsprojects"
 } else {
   config_h_dir = "."
 }
 
@@ skipping 12 matching lines @@
     cflags = [ "/wd4267" ]
   }
 }
 
 if (component_mode == "shared_library") {
   config("protobuf_use_dlls") {
     defines = [ "PROTOBUF_USE_DLLS" ]
   }
 }
 
-# This condif should be applied to targets using generated code from the proto
+# This config should be applied to targets using generated code from the proto
 # compiler. It sets up the include directories properly.
 config("using_proto") {
-  include_dirs = [ "$root_gen_dir/protoc_out" ]
+  include_dirs = [
+    "src",
+    "$root_gen_dir/protoc_out",
+  ]
 }
 
 protobuf_lite_sources = [
   "src/google/protobuf/extension_set.cc",
   "src/google/protobuf/extension_set.h",
   "src/google/protobuf/generated_message_util.cc",
   "src/google/protobuf/generated_message_util.h",
   "src/google/protobuf/io/coded_stream.cc",
   "src/google/protobuf/io/coded_stream.h",
   "src/google/protobuf/io/coded_stream_inl.h",
@@ skipping 202 matching lines @@
     }
 
     cflags = protobuf_lite_cflags
 
     deps = [
       ":protobuf_full",
     ]
     deps += [ "//build/config/sanitizers:deps" ]
   }
 }