[tracing] Implement composable memory usage estimators.

base/trace_event/memory_usage_estimator.cc

Index: base/trace_event/memory_usage_estimator.cc
diff --git a/base/trace_event/memory_usage_estimator.cc b/base/trace_event/memory_usage_estimator.cc
new file mode 100644
index 0000000000000000000000000000000000000000..c769d5b6f1e5bc169e127508ba3bb47deb56e91d
--- /dev/null
+++ b/base/trace_event/memory_usage_estimator.cc
@@ -0,0 +1,14 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/trace_event/memory_usage_estimator.h"
+
+namespace base {
+namespace trace_event {
+
+template size_t EstimateMemoryUsage(const std::string&);
+template size_t EstimateMemoryUsage(const string16&);
+
+}  // namespace trace_event
+}  // namespace base