Add SkCachedData and use it for SkMipMap

src/core/SkCachedData.h

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkCachedData_DEFINED
+#define SkCachedData_DEFINED
+
+#include "SkTypes.h"
+
+class SkDiscardableMemory;
+
+class SkCachedData : ::SkNoncopyable {
+public:
+    static SkCachedData* Create(size_t bytes, SkDiscardableMemory* = NULL);
+
+    virtual ~SkCachedData() {}
+
+    size_t size() const { return fSize; }
+    const void* data() const { return fData; }
+
+    void* writable_data() { return fData; }
+
+    void ref();
+    void unref();
+
+    int testing_only_getRefCnt() const { return fRefCnt >> 1; }
+    bool testing_only_isInCache() const { return fRefCnt & 1; }
+
+protected:
+    SkCachedData(void* data, size_t size);
+
+    virtual void onLock() = 0;      // called when owner count rises above 1
+    virtual void onUnlock() = 0;    // called when owner count falls back to 1
+
+    void setData(void* data) { fData = data; }
+
+private:
+    void*   fData;
+    size_t  fSize;
+    int32_t fRefCnt;    // low-bit means we're owned by the cache
+
+    void lock() {
+        this->onLock();
+    }
+
+    void unlock() {
+        this->onUnlock();
+        fData = NULL;   // signal that we're unlocked
+    }
+
+public:
+    void ref_from_cache();
+    void unref_from_cache();
+};
+
+#endif