Enable generating SkImage with the same uniqueID in SkImageGenerator subclass

include/core/SkImageGenerator.h

Index: include/core/SkImageGenerator.h
diff --git a/include/core/SkImageGenerator.h b/include/core/SkImageGenerator.h
index 1a46f6b9cd7dfc39c753170b7da6046ed0877bb2..3712a924a55f45e21439f9d631bad4ad02ecd3f9 100644
--- a/include/core/SkImageGenerator.h
+++ b/include/core/SkImageGenerator.h
@@ -251,7 +251,11 @@ public:
     }
 
 protected:
-    SkImageGenerator(const SkImageInfo& info);
+    enum {
+        kNeedNewImageUniqueID = 0
+    };
+
+    SkImageGenerator(const SkImageInfo& info, uint32_t uniqueId = kNeedNewImageUniqueID);
 
     virtual SkData* onRefEncodedData(SK_REFENCODEDDATA_CTXPARAM);