SkImage method for detecting lazy decoding

BUG=skia:4224
R=reed@google.com

Review URL: https://codereview.chromium.org/1305453007
diff --git a/src/lazy/SkDiscardablePixelRef.h b/src/lazy/SkDiscardablePixelRef.h
index e8451ee..3b38fa6 100644
--- a/src/lazy/SkDiscardablePixelRef.h
+++ b/src/lazy/SkDiscardablePixelRef.h
@@ -36,6 +36,8 @@
         return fGenerator->refEncodedData();
     }
 
+    bool onIsLazyGenerated() const override { return true; }
+
 private:
     SkImageGenerator* const fGenerator;
     SkDiscardableMemory::Factory* const fDMFactory;