Standardize "net" category trace events

net/base/trace_constants.h

Index: net/base/trace_constants.h
diff --git a/net/base/trace_constants.h b/net/base/trace_constants.h
new file mode 100644
index 0000000000000000000000000000000000000000..dc5d84aabc21a8b4943fc6dfb826f5828af5bd9e
--- /dev/null
+++ b/net/base/trace_constants.h
@@ -0,0 +1,15 @@
+// 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.
+
+#ifndef NET_BASE_TRACE_CONSTANTS_H_
+#define NET_BASE_TRACE_CONSTANTS_H_
+
+namespace net {
+
+// Net Category used in Tracing.
+extern const char kNetTracingCategory[];
+
+}  // namespace net
+
+#endif  // NET_BASE_TRACE_CONSTANTS_H_