add newImage API

BUG=skia:3277
related bug: skbug.com/3276

Review URL: https://codereview.chromium.org/821083002
diff --git a/src/image/SkImage_Base.h b/src/image/SkImage_Base.h
index 8c1dfad..512c80c 100644
--- a/src/image/SkImage_Base.h
+++ b/src/image/SkImage_Base.h
@@ -59,6 +59,10 @@
                                   SkShader::TileMode,
                                   const SkMatrix* localMatrix) const { return NULL; };
 
+    // newWidth > 0, newHeight > 0, subset either NULL or a proper subset of this bounds
+    virtual SkImage* onNewImage(int newWidth, int newHeight, const SkIRect* subset,
+                                SkFilterQuality) const;
+
 private:
     const SkSurfaceProps fProps;