Consolidate PixelRef flattables with the standard impl

The flatten method on these functions can no longer be
const as SkFlattenables declaration is not const and 
would result in the const methods only being called when
the reference to the object was const.
Review URL: https://codereview.appspot.com/5941043

git-svn-id: http://skia.googlecode.com/svn/trunk@3533 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/images/SkImageRef.cpp b/src/images/SkImageRef.cpp
index 1d6b270..cdd80e7 100644
--- a/src/images/SkImageRef.cpp
+++ b/src/images/SkImageRef.cpp
@@ -189,7 +189,7 @@
     fFactory = NULL;
 }
 
-void SkImageRef::flatten(SkFlattenableWriteBuffer& buffer) const {
+void SkImageRef::flatten(SkFlattenableWriteBuffer& buffer) {
     this->INHERITED::flatten(buffer);
 
     buffer.write8(fConfig);