Fix minor nits in SnapshotClocks() and fix mac build

Followup to aosp/959875. It just gets rid of the default argument
(seemed unjustified) and makes the argument name consistent.
Also make the behavior consistent on mac os, mainly to get rid of
the unused variable warning due to the #ifdef.

Bug: 130543265
Test: N/A, non functional refactoring.
Change-Id: I9e29f9e475a3863450a6ec1fe50e375c19a63406
diff --git a/src/tracing/core/tracing_service_impl.h b/src/tracing/core/tracing_service_impl.h
index 40cb924..fbae3b6 100644
--- a/src/tracing/core/tracing_service_impl.h
+++ b/src/tracing/core/tracing_service_impl.h
@@ -496,7 +496,7 @@
                                TracingSession* tracing_session,
                                DataSourceInstance* instance);
   void SnapshotSyncMarker(std::vector<TracePacket>*);
-  void SnapshotClocks(std::vector<TracePacket>*, bool set_timestamp = true);
+  void SnapshotClocks(std::vector<TracePacket>*, bool set_root_timestamp);
   void SnapshotStats(TracingSession*, std::vector<TracePacket>*);
   TraceStats GetTraceStats(TracingSession* tracing_session);
   void MaybeEmitTraceConfig(TracingSession*, std::vector<TracePacket>*);