don't initialize the default Skia tracer if one has already been provided

src/utils/SkEventTracer.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkEventTracer.h"
 #include "SkOnce.h"
 
@@ skipping 20 matching lines @@
     };
     virtual const char* getCategoryGroupName(
       const uint8_t* category_group_enabled) SK_OVERRIDE {
         static const char* dummy = "dummy";
         return dummy;
     };
 };
 
 SkEventTracer *SkEventTracer::gInstance;
 
-static void intiailize_default_tracer(int) {
-    SkEventTracer::SetInstance(SkNEW(SkDefaultEventTracer));
+static void intialiize_default_tracer(void *current_instance) {
+    if (NULL == current_instance) {
+        SkEventTracer::SetInstance(SkNEW(SkDefaultEventTracer));
+    }
 }
 
 static void cleanup_tracer() {
     // calling SetInstance will delete the existing instance.
     SkEventTracer::SetInstance(NULL);
 }
 
 SkEventTracer* SkEventTracer::GetInstance() {
     SK_DECLARE_STATIC_ONCE(once);
-    SkOnce(&once, intiailize_default_tracer, 0, cleanup_tracer);
+    SkOnce(&once,
+           intialiize_default_tracer,
+           SkEventTracer::gInstance,
+           cleanup_tracer);
     SkASSERT(NULL != SkEventTracer::gInstance);
     return SkEventTracer::gInstance;
 }