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

base/trace_event/trace_event.h

Index: base/trace_event/trace_event.h
diff --git a/base/trace_event/trace_event.h b/base/trace_event/trace_event.h
index a2fb97c9932c3b0df7d8ff7db2eb29f1d7286c9e..f5e7ff5fd1333043d4d96dd7cec9f4e3a958bce0 100644
--- a/base/trace_event/trace_event.h
+++ b/base/trace_event/trace_event.h
@@ -917,6 +917,10 @@
       } \
     } 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 { \