perfetto: fix Chromium build

Change-Id: Ifff36d23b8c9b2f9df6e4bae43a9535a3fee4b64
diff --git a/src/tracing/core/service_impl_unittest.cc b/src/tracing/core/service_impl_unittest.cc
index 2ffe0bd..8787672 100644
--- a/src/tracing/core/service_impl_unittest.cc
+++ b/src/tracing/core/service_impl_unittest.cc
@@ -768,7 +768,7 @@
     ASSERT_TRUE(trace_partition.ParseFromArray(trace_raw.data() + start, size));
     merged_trace.MergeFrom(trace_partition);
   }
-  EXPECT_GE(num_markers, kNumMarkers);
+  EXPECT_GE(num_markers, static_cast<size_t>(kNumMarkers));
 
   protos::Trace whole_trace;
   ASSERT_TRUE(whole_trace.ParseFromString(trace_raw));