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 58bd02370bff9bbefb4f02c1aa5c4535d33112b4..9c6e71da3d4fc858094d8dbe19b025276dabb1dc 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:
+//
+// TRACE_EVENT_INSTANT0("MY_SUBSYSTEM_1, MY_SUB_SYSTEM_2", "ImportantEvent")
+//
+// 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",
 //                  "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"));
 //
 //