[googletest] Add a gtest_prod target.

This allows including "gtest_prod.h" for the FRIEND_TEST macro.

Change-Id: Id2890a915fedd5e134f867ef280f4cf1c5f2521d
diff --git a/BUILD.gn b/BUILD.gn
index 50d1ab2..cca813d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -72,6 +72,15 @@
   }
 }
 
+# Library that defines the FRIEND_TEST macro.
+source_set("gtest_prod") {
+  testonly = false
+  public = [
+    "googletest/include/gtest/gtest_prod.h",
+  ]
+  public_configs = [ ":gtest_config" ]
+}
+
 static_library("gtest_main") {
   testonly = true
   sources = [