Relocate anisotropic mipmap logic to SkMipMap::extractLevel()

src/core/SkMipMap.cpp

Index: src/core/SkMipMap.cpp
diff --git a/src/core/SkMipMap.cpp b/src/core/SkMipMap.cpp
index 08602b7e00728b100d78862f76288a6afd4770f1..7266dc6b21192c16019c0277ea4387244033d209 100644
--- a/src/core/SkMipMap.cpp
+++ b/src/core/SkMipMap.cpp
@@ -320,11 +320,15 @@ SkMipMap* SkMipMap::Build(const SkPixmap& src, SkDiscardableFactoryProc fact) {
 
 ///////////////////////////////////////////////////////////////////////////////
 
-bool SkMipMap::extractLevel(SkScalar scale, Level* levelPtr) const {
+bool SkMipMap::extractLevel(const SkSize& scaleSize, Level* levelPtr) const {
     if (nullptr == fLevels) {
         return false;
     }
 
+    SkASSERT(scaleSize.width() >= 0 && scaleSize.height() >= 0);
+    // Use the smallest scale to match the GPU impl.
+    const SkScalar scale = SkTMin(scaleSize.width(), scaleSize.height());
+
     if (scale >= SK_Scalar1 || scale <= 0 || !SkScalarIsFinite(scale)) {
         return false;
     }