memory-infra: Flush trace writer when Flush is called

The memory infra proto writer will flush the trace writer when
flush is called.

Change-Id: I47d9d287c339b513ff5a5b492640c0df3482573d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2557880Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: ssid <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830650}

services/resource_coordinator/public/cpp/memory_instrumentation/tracing_observer_proto.cc

@@ -156,7 +156,11 @@ void TracingObserverProto::StopTracing(
 }
 
 void TracingObserverProto::Flush(
-    base::RepeatingClosure flush_complete_callback) {}
+    base::RepeatingClosure flush_complete_callback) {
+  base::AutoLock lock(producer_lock_);
+  if (trace_writer_)
+    trace_writer_->Flush();
+}
 
 void TracingObserverProto::MemoryMapsAsProtoInto(
     const std::vector<mojom::VmRegionPtr>& memory_maps,