Reland of SkOncePtr -> SkOnce

src/utils/SkEventTracer.cpp

Index: src/utils/SkEventTracer.cpp
diff --git a/src/utils/SkEventTracer.cpp b/src/utils/SkEventTracer.cpp
index 32a0207c23ebaa0f3d0ac9edd7a29688a67c0e23..0a748d1464d70f9d30ac9d66f5909ad798cb2a2d 100644
--- a/src/utils/SkEventTracer.cpp
+++ b/src/utils/SkEventTracer.cpp
@@ -7,7 +7,7 @@
 
 #include "SkAtomics.h"
 #include "SkEventTracer.h"
-#include "SkOncePtr.h"
+#include "SkOnce.h"
 
 #include <stdlib.h>
 
@@ -39,9 +39,8 @@
     }
 };
 
-// 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));
@@ -54,5 +53,8 @@
     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;
 }