Add getting the size of a given mipmap level.

src/core/SkMipMap.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkMipMap_DEFINED
 #define SkMipMap_DEFINED
 
 #include "SkCachedData.h"
 #include "SkPixmap.h"
 #include "SkScalar.h"
 #include "SkSize.h"
 
 class SkBitmap;
 class SkDiscardableMemory;
 
 typedef SkDiscardableMemory* (*SkDiscardableFactoryProc)(size_t bytes);
 
 class SkMipMap : public SkCachedData {
 public:
     static SkMipMap* Build(const SkPixmap& src, SkDiscardableFactoryProc);
     static SkMipMap* Build(const SkBitmap& src, SkDiscardableFactoryProc);
 
-    // This function lets you determine how many levels a SkMipMap will have without
-    // creating that mipmap.
+    // Determines how many levels a SkMipMap will have without creating that mipmap.
     static int ComputeLevelCount(int baseWidth, int baseHeight);
 
+    // Determines the size of a given mipmap level.
+    static SkSize ComputeLevelSize(int baseWidth, int baseHeight, int level);
+
     struct Level {
         SkPixmap    fPixmap;
         SkSize      fScale; // < 1.0
     };
 
     bool extractLevel(const SkSize& scale, Level*) const;
     int countLevels() const;
     bool getLevel(int index, Level*) const;
 
 protected:
     void onDataChange(void* oldData, void* newData) override {
         fLevels = (Level*)newData; // could be nullptr
     }
 
 private:
     Level*  fLevels;
     int     fCount;
 
     // we take ownership of levels, and will free it with sk_free()
     SkMipMap(void* malloc, size_t size) : INHERITED(malloc, size) {}
     SkMipMap(size_t size, SkDiscardableMemory* dm) : INHERITED(size, dm) {}
 
     static size_t AllocLevelsSize(int levelCount, size_t pixelSize);
 
     typedef SkCachedData INHERITED;
 };
 
 #endif