Allow tracing categories to default to disabled.

base/debug/trace_event.h

 // Copyright (c) 2012 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.
 
 // This header file defines the set of trace_event macros without specifying
 // how the events actually get collected and stored. If you need to expose trace
 // events to some other universe, you can copy-and-paste this file as well as
 // trace_event.h, modifying the macros contained there as necessary for the
 // target platform. The end result is that multiple libraries can funnel events
 // through to a shared trace event collector.
@@ skipping 182 matching lines @@
 
 #include "base/atomicops.h"
 #include "base/debug/trace_event_impl.h"
 #include "build/build_config.h"
 
 // By default, const char* argument values are assumed to have long-lived scope
 // and will not be copied. Use this macro to force a const char* to be copied.
 #define TRACE_STR_COPY(str) \
     trace_event_internal::TraceStringWithCopy(str)
 
+// This will mark the trace event as disabled by default. The user will need
+// to explicitly enable the event.
+#define TRACE_DISABLED_BY_DEFAULT(name) "disabled-by-default-" name
+
 // By default, uint64 ID argument values are not mangled with the Process ID in
 // TRACE_EVENT_ASYNC macros. Use this macro to force Process ID mangling.
 #define TRACE_ID_MANGLE(id) \
     trace_event_internal::TraceID::ForceMangle(id)
 
 // By default, pointers are mangled with the Process ID in TRACE_EVENT_ASYNC
 // macros. Use this macro to prevent Process ID mangling.
 #define TRACE_ID_DONT_MANGLE(id) \
     trace_event_internal::TraceID::DontMangle(id)
 
@@ skipping 1116 matching lines @@
   const char* name_;
   IDType id_;
 
   DISALLOW_COPY_AND_ASSIGN(TraceScopedTrackableObject);
 };
 
 } // namespace debug
 } // namespace base
 
 #endif /* BASE_DEBUG_TRACE_EVENT_H_ */