Allow use of the chrome trace categories string from the tracing ui internals.

content/public/browser/trace_controller.h

Index: content/public/browser/trace_controller.h
diff --git a/content/public/browser/trace_controller.h b/content/public/browser/trace_controller.h
index 49670f240aba4405c24f0211827b94a4bcbdc9ec..0558363300a9d8c0c9d5911da48d1b954dfa925d 100644
--- a/content/public/browser/trace_controller.h
+++ b/content/public/browser/trace_controller.h
@@ -24,14 +24,12 @@ class TraceController {
   // Currently only one subscriber is allowed at a time.
   // Tracing begins immediately locally, and asynchronously on child processes
   // as soon as they receive the BeginTracing request.
-  // By default, all categories are traced except those matching "test_*".
   //
   // If BeginTracing was already called previously,
   //   or if an EndTracingAsync is pending,
   //   or if another subscriber is tracing,
   //   BeginTracing will return false meaning it failed.
-  virtual bool BeginTracing(TraceSubscriber* subscriber) = 0;
-
+  //
   // |categories| is a comma-delimited list of category wildcards.
   // A category can have an optional '-' prefix to make it an excluded category.
   // All the same rules apply above, so for example, having both included and