fix the test error

src/core/ext/transport/cronet/transport/cronet_transport.c

@@ -855,7 +855,7 @@ static enum e_op_result execute_stream_op(grpc_exec_ctx *exec_ctx,
     s->header_array.capacity = s->header_array.count;
     CRONET_LOG(GPR_DEBUG, "bidirectional_stream_start(%p, %s)", s->cbs, url);
     bidirectional_stream_start(s->cbs, url, 0, method, &s->header_array, false);
-    int header_index;
+    unsigned int header_index;
     for (header_index = 0; header_index < s->header_array.count;
          header_index++) {
       gpr_free((void *)s->header_array.headers[header_index].key);