Trace category groups and category filter.

base/debug/trace_event.h

Index: base/debug/trace_event.h
diff --git a/base/debug/trace_event.h b/base/debug/trace_event.h
index fc8897e1e73cf5e928a72552a24a604ed16ba2a5..9a1ed7488e5577c0d65b13def5412bf30d2a2e5b 100644
--- a/base/debug/trace_event.h
+++ b/base/debug/trace_event.h
@@ -19,6 +19,13 @@
 // implicitly with a string. For example:
 //   TRACE_EVENT_INSTANT0("MY_SUBSYSTEM", "SomeImportantEvent")
 //
+// A category can be made up of several tags, for example:

[nduca, 2013/01/28 08:15:28]

The first argument to traces is a comma-separated list of categories, for
example:

Sorry to change my mind on this since the last review, but now that I see this
patch, its starting to look clearer. I think we should try to go through and
where we say Category, instead say CategoryGroup. Category just means "tag", and
we avoid the word CategoryGroup to indicate cat1,cat2 type things.

+//
+// TRACE_EVENT_INSTANT0("MY_SUBSYSTEM_1, MY_SUB_SYSTEM_2", "ImportantEvent")

[nduca, 2013/01/28 08:15:28]

Lets give a concerete example here so its less abstract. So you could give an
example like "MouseMove" which is of interest to both Views and Rendering.

It is often the case that one trace may belong in multiple categories at the
same time. The first argument to the trace can be a comma-separated list of
categories, forming a category group, like so:
<example, "Input,Views" maybe.>

Once instrumente in this fashion, the trace event can be enabled or disabled by
either asking for Input, Views, or both.

+//
+// We can enable/disable tracing of ImportantEvent by enabling/disabling either
+// tag.
+//
 // Events can be INSTANT, or can be pairs of BEGIN and END in the same scope:
 //   TRACE_EVENT_BEGIN0("MY_SUBSYSTEM", "SomethingCostly")
 //   doSomethingCostly()
@@ -101,7 +108,7 @@
 // Tracing copies the pointers, not the string content, of the strings passed
 // in for category, name, and arg_names.  Thus, the following code will
 // cause problems:
-//     char* str = strdup("impprtantName");
+//     char* str = strdup("importantName");
 //     TRACE_EVENT_INSTANT0("SUBSYSTEM", str);  // BAD!
 //     free(str);                   // Trace system now has dangling pointer
 //
@@ -113,13 +120,13 @@
 //
 // When are string argument values copied:
 // const char* arg_values are only referenced by default:
-//     TRACE_EVENT1("category", "name",
+//     TRACE_EVENT1("category_tag1", "name",

[nduca, 2013/01/28 08:15:28]

i think its safe to say category once we make the previously mentioned changes

 //                  "arg1", "literal string is only referenced");
 // Use TRACE_STR_COPY to force copying of a const char*:
-//     TRACE_EVENT1("category", "name",
+//     TRACE_EVENT1("category_tag1", "name",
 //                  "arg1", TRACE_STR_COPY("string will be copied"));
 // std::string arg_values are always copied:
-//     TRACE_EVENT1("category", "name",
+//     TRACE_EVENT1("category_tag1", "name",
 //                  "arg1", std::string("string will be copied"));
 //
 //