Adding Flush() to TraceWriter

Exposes an API to explicitly flush data from
TraceWriter. This will release the current chunk
and send an IPC to the service.
It's quite useful for tests.
Unrelatedly, fix unittests failure on mac.

Bug: 74380167
Change-Id: I5a03e411cedf993094d642ca6da1a873afce885e
diff --git a/src/tracing/core/trace_writer_impl_unittest.cc b/src/tracing/core/trace_writer_impl_unittest.cc
index 126a11d..504ff89 100644
--- a/src/tracing/core/trace_writer_impl_unittest.cc
+++ b/src/tracing/core/trace_writer_impl_unittest.cc
@@ -105,6 +105,7 @@
 }
 
 // TODO(primiano): add multi-writer test.
+// TODO(primiano): add Flush() test.
 
 }  // namespace
 }  // namespace perfetto