Reland of SkOncePtr -> SkOnce

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 "SkAtomics.h"
 #include "SkEventTracer.h"
-#include "SkOncePtr.h"
+#include "SkOnce.h"
 
 #include <stdlib.h>
 
 class SkDefaultEventTracer : public SkEventTracer {
     SkEventTracer::Handle
         addTraceEvent(char phase,
                       const uint8_t* categoryEnabledFlag,
                       const char* name,
                       uint64_t id,
                       int numArgs,
@@ skipping 11 matching lines @@
         static uint8_t no = 0;
         return &no;
     }
     const char* getCategoryGroupName(
       const uint8_t* categoryEnabledFlag) override {
         static const char* dummy = "dummy";
         return dummy;
     }
 };
 
-// We prefer gUserTracer if it's been set, otherwise we fall back on gDefaultTracer.
+// We prefer gUserTracer if it's been set, otherwise we fall back on a default tracer;
 static SkEventTracer* gUserTracer = nullptr;
-SK_DECLARE_STATIC_ONCE_PTR(SkDefaultEventTracer, gDefaultTracer);
 
 void SkEventTracer::SetInstance(SkEventTracer* tracer) {
     SkASSERT(nullptr == sk_atomic_load(&gUserTracer, sk_memory_order_acquire));
     sk_atomic_store(&gUserTracer, tracer, sk_memory_order_release);
     // An atomic load during process shutdown is probably overkill, but safe overkill.
     atexit([]() { delete sk_atomic_load(&gUserTracer, sk_memory_order_acquire); });
 }
 
 SkEventTracer* SkEventTracer::GetInstance() {
     if (SkEventTracer* tracer = sk_atomic_load(&gUserTracer, sk_memory_order_acquire)) {
         return tracer;
     }
-    return gDefaultTracer.get([]{ return new SkDefaultEventTracer; });
+    static SkOnce once;
+    static SkDefaultEventTracer* defaultTracer;
+    once([] { defaultTracer = new SkDefaultEventTracer; });
+    return defaultTracer;
 }