change setAlphaType to not modify the pixelref's genID

include/core/SkPixelRef.h

Index: include/core/SkPixelRef.h
diff --git a/include/core/SkPixelRef.h b/include/core/SkPixelRef.h
index 62edc3ec954a1dbc6d3a2c0e889f3db20cb7776f..5a9a5a05f7375406f5b0e2f03baafaef668c8769 100644
--- a/include/core/SkPixelRef.h
+++ b/include/core/SkPixelRef.h
@@ -130,15 +130,15 @@ public:
      *  Call this if you have changed the contents of the pixels. This will in-
      *  turn cause a different generation ID value to be returned from
      *  getGenerationID().
-     *
-     *  If the alphatype has also changed, specify its new value as well. If
-     *  the new pixels' alphatype is the same, this can be called with no
-     *  parameter.
      */
-    void notifyPixelsChanged(SkAlphaType);
-    void notifyPixelsChanged() {
-        this->notifyPixelsChanged(fInfo.fAlphaType);
-    }
+    void notifyPixelsChanged();
+
+    /**
+     *  Change the info's AlphaType. Note that this does not automatically
+     *  invalidate the generation ID. If the pixel values themselves have
+     *  changed, then you must explicitly call notifyPixelsChanged() as well.
+     */
+    void changeAlphaType(SkAlphaType at);
 
     /** Returns true if this pixelref is marked as immutable, meaning that the
         contents of its pixels will not change for the lifetime of the pixelref.