Clean up gtest_prod mess.

Change-Id: I7b9b800c65cfc918770cd54b77993d2f0f2c0df6
diff --git a/aos/common/gtest_prod.h b/aos/common/gtest_prod.h
index fe0b056..6c7d96e 100644
--- a/aos/common/gtest_prod.h
+++ b/aos/common/gtest_prod.h
@@ -1,8 +1,6 @@
 #ifndef AOS_COMMON_GTEST_PROD_H_
 #define AOS_COMMON_GTEST_PROD_H_
 
-#include "gtest/gtest_prod.h"
-
 // These macros replace gtest's FRIEND_TEST if the test is in a different
 // namespace than the code that needs to make it a friend.
 // Example:
@@ -34,4 +32,8 @@
 #define FRIEND_TEST_NAMESPACE(test_case_name, test_name, namespace_name) \
     friend class namespace_name::test_case_name##_##test_name##_Test
 
+// Copied from googletest's gtest_prod.h. See that file for documentation.
+#define FRIEND_TEST(test_case_name, test_name) \
+  friend class test_case_name##_##test_name##_Test
+
 #endif  // AOS_COMMON_GTEST_PROD_H_