Fix build using system clang.

Remove pipestats as it is unused.

Change-Id: I6e3e3c47b6f4562611f5f87b5b9ebd561f9ec033
diff --git a/src/tracing/core/service_impl_unittest.cc b/src/tracing/core/service_impl_unittest.cc
index 9b50bda..accb28c 100644
--- a/src/tracing/core/service_impl_unittest.cc
+++ b/src/tracing/core/service_impl_unittest.cc
@@ -821,7 +821,7 @@
   const int kNumMarkers = 5;
   auto writer = producer->CreateTraceWriter("data_source");
   for (int i = 1; i <= 100; i++) {
-    std::string payload(i, 'A' + (i % 25));
+    std::string payload(static_cast<size_t>(i), 'A' + (i % 25));
     writer->NewTracePacket()->set_for_testing()->set_str(payload.c_str());
     if (i % (100 / kNumMarkers) == 0) {
       writer->Flush();