process stats: clear more incremental state, and write marker

Plus addressing leftover comments from previous CLs in the chain.

I've skimped out on adding more unittests for the process stats clearing. WDYT, should I?

Bug: 132394650
Change-Id: I9f891d11508e21bcbbf21531f5920b3a4fcde1f7
diff --git a/src/tracing/core/tracing_service_impl_unittest.cc b/src/tracing/core/tracing_service_impl_unittest.cc
index 74653c6..7442fef 100644
--- a/src/tracing/core/tracing_service_impl_unittest.cc
+++ b/src/tracing/core/tracing_service_impl_unittest.cc
@@ -1629,8 +1629,8 @@
   producer->RegisterDataSource("ds_incremental2", false, false,
                                /*handles_incremental_state_clear=*/true);
 
-  // Data source that does *not* advertise itself as support incremental state
-  // clears.
+  // Data source that does *not* advertise itself as supporting incremental
+  // state clears.
   producer->RegisterDataSource("ds_selfcontained", false, false,
                                /*handles_incremental_state_clear=*/false);