Add getting the size of a given mipmap level.

src/core/SkMipMap.h

Index: src/core/SkMipMap.h
diff --git a/src/core/SkMipMap.h b/src/core/SkMipMap.h
index 928c12d4369dd0a0e646c0fbe68ee276c373e251..5cb4ea8ce670b30c90eec35455b65befd5c65d0c 100644
--- a/src/core/SkMipMap.h
+++ b/src/core/SkMipMap.h
@@ -23,10 +23,12 @@ 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