media: Always enable trace event for "media" category group.

base/trace_event/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 899 matching lines @@
 #define TRACE_EVENT_CATEGORY_GROUP_ENABLED(category_group, ret) \
     do { \
       INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO(category_group); \
       if (INTERNAL_TRACE_EVENT_CATEGORY_GROUP_ENABLED_FOR_RECORDING_MODE()) { \
         *ret = true; \
       } else { \
         *ret = false; \
       } \
     } while (0)
 
+// Macro to explicitly enable a given category group.

[nduca, 2015/07/23 17:00:53]

Can you add some explanation text on when you have to use this versus when you
don't? I think some folks could misread this and think you always have to init
categories. 

wdyt about TRACE_EVENT_WARMUP_CATEGORY as a name so its usage is clearer.

[xhwang, 2015/07/23 22:24:08]

Done.

+#define TRACE_EVENT_INIT_CATEGORY(category_group) \
+  INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO(category_group)
+
 // Macro to efficiently determine, through polling, if a new trace has begun.
 #define TRACE_EVENT_IS_NEW_TRACE(ret) \
     do { \
       static int INTERNAL_TRACE_EVENT_UID(lastRecordingNumber) = 0; \
       int num_traces_recorded = TRACE_EVENT_API_GET_NUM_TRACES_RECORDED(); \
       if (num_traces_recorded != -1 && \
           num_traces_recorded != \
           INTERNAL_TRACE_EVENT_UID(lastRecordingNumber)) { \
         INTERNAL_TRACE_EVENT_UID(lastRecordingNumber) = \
             num_traces_recorded; \
@@ skipping 816 matching lines @@
   const char* name_;
   IDType id_;
 
   DISALLOW_COPY_AND_ASSIGN(TraceScopedTrackableObject);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_TRACE_EVENT_H_