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

chrome/browser/ui/webui/tracing_ui.cc

Index: chrome/browser/ui/webui/tracing_ui.cc
diff --git a/chrome/browser/ui/webui/tracing_ui.cc b/chrome/browser/ui/webui/tracing_ui.cc
index 2cad8be3ad35a46524b04c94d30b2dd6f15cad57..3808fb286f74968661de4d8426f57097cf565d5b 100644
--- a/chrome/browser/ui/webui/tracing_ui.cc
+++ b/chrome/browser/ui/webui/tracing_ui.cc
@@ -431,18 +431,22 @@ void TracingMessageHandler::SaveTraceFileComplete() {
 
 void TracingMessageHandler::OnBeginTracing(const ListValue* args) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
-  DCHECK(args->GetSize() == 1);
+  DCHECK_EQ(args->GetSize(), (size_t) 2);
 
   bool system_tracing_requested = false;
   bool ok = args->GetBoolean(0, &system_tracing_requested);
   DCHECK(ok);
 
+  std::string chrome_categories;
+  ok = args->GetString(1, &chrome_categories);
+  DCHECK(ok);
+
   trace_enabled_ = true;
   // TODO(jbates) This may fail, but that's OK for current use cases.
   //              Ex: Multiple about:gpu traces can not trace simultaneously.
   // TODO(nduca) send feedback to javascript about whether or not BeginTracing
   //             was successful.
-  TraceController::GetInstance()->BeginTracing(this);
+  TraceController::GetInstance()->BeginTracing(this, chrome_categories);
 
   if (system_tracing_requested) {
 #if defined(OS_CHROMEOS)