SkOnce: add option to call another cleanup function once at exit.

src/core/SkData.cpp

Index: src/core/SkData.cpp
diff --git a/src/core/SkData.cpp b/src/core/SkData.cpp
index fd963a9ff50b22aedf2d18f30ee6880e08afd598..176e93c47be2f98303ba849cbb0eda597d370e6c 100644
--- a/src/core/SkData.cpp
+++ b/src/core/SkData.cpp
@@ -48,16 +48,18 @@ size_t SkData::copyRange(size_t offset, size_t length, void* buffer) const {
 
 ///////////////////////////////////////////////////////////////////////////////
 
-void SkData::NewEmptyImpl(SkData** empty) {
-    *empty = new SkData(NULL, 0, NULL, NULL);
+static SkData* gEmptyDataRef = NULL;
+static void cleanup_gEmptyDataRef() { gEmptyDataRef->unref(); }
+
+void SkData::NewEmptyImpl(int) {
+    gEmptyDataRef = new SkData(NULL, 0, NULL, NULL);
 }
 
 SkData* SkData::NewEmpty() {
-    static SkData* gEmptyRef;
     SK_DECLARE_STATIC_ONCE(once);
-    SkOnce(&once, SkData::NewEmptyImpl, &gEmptyRef);
-    gEmptyRef->ref();
-    return gEmptyRef;
+    SkOnce(&once, SkData::NewEmptyImpl, 0, cleanup_gEmptyDataRef);
+    gEmptyDataRef->ref();
+    return gEmptyDataRef;
 }
 
 // assumes fPtr was allocated via sk_malloc