Expressed the thread-safety annotations in code, replacing the existing comment-based system (by Aaron Jacobs).

include/gtest/internal/gtest-port.h

@@ -1789,6 +1789,10 @@ typedef TypeWithSize<8>::Int TimeInMillis;  // Represents time in milliseconds.
 #define GTEST_DEFINE_string_(name, default_val, doc) \
     GTEST_API_ ::testing::internal::String GTEST_FLAG(name) = (default_val)
 
+// Thread annotations
+#define GTEST_EXCLUSIVE_LOCK_REQUIRED_(locks)
+#define GTEST_LOCK_EXCLUDED_(locks)
+
 // Parses 'str' for a 32-bit signed integer.  If successful, writes the result
 // to *value and returns true; otherwise leaves *value unchanged and returns
 // false.