Fix chromium roll

It's failing with

error: comparison of integers of different signs
https://ci.chromium.org/p/chromium/builders/luci.chromium.try/linux_chromium_rel_ng/246027
Change-Id: Ia5dee5a90a748802043a143b867d2179855a8cc4
diff --git a/src/tracing/core/service_impl_unittest.cc b/src/tracing/core/service_impl_unittest.cc
index 03c598e..812a91b 100644
--- a/src/tracing/core/service_impl_unittest.cc
+++ b/src/tracing/core/service_impl_unittest.cc
@@ -941,7 +941,7 @@
   ds_config23->set_target_buffer(2);  // same buffer as data_source2.2.
   consumer->EnableTracing(trace_config);
 
-  ASSERT_EQ(3, tracing_session()->num_buffers());
+  ASSERT_EQ(3u, tracing_session()->num_buffers());
   std::set<BufferID> expected_buffers_producer1 = {
       tracing_session()->buffers_index[0]};
   std::set<BufferID> expected_buffers_producer2 = {