fix fuzzer build & signedness of a test comparison (for chrome)

Change-Id: I27f831e0a5a79a5a4761698a586299f18f5311a1
diff --git a/src/tracing/core/tracing_service_impl_unittest.cc b/src/tracing/core/tracing_service_impl_unittest.cc
index 781d672..51d0fd1 100644
--- a/src/tracing/core/tracing_service_impl_unittest.cc
+++ b/src/tracing/core/tracing_service_impl_unittest.cc
@@ -1668,7 +1668,7 @@
   DataSourceInstanceID ds_incremental2 =
       producer->GetDataSourceInstanceId("ds_incremental2");
 
-  const int kNumClears = 3;
+  const size_t kNumClears = 3;
   std::function<void()> checkpoint =
       task_runner.CreateCheckpoint("clears_received");
   std::vector<std::vector<DataSourceInstanceID>> clears_seen;