Don't serialize anything for SkEmptyShader.

src/core/SkEmptyShader.h

Index: src/core/SkEmptyShader.h
diff --git a/src/core/SkEmptyShader.h b/src/core/SkEmptyShader.h
index 8809d5f958b2a170ed866d1c252436df21001660..7b87c284f7f4a3f8fd6e80dc04d0a9ec8314b906 100644
--- a/src/core/SkEmptyShader.h
+++ b/src/core/SkEmptyShader.h
@@ -34,6 +34,12 @@ protected:
         return NULL;
     }
 
+    void flatten(SkWriteBuffer& buffer) const override {
+        // Do nothing.
+        // We just don't want to fall through to SkShader::flatten(),
+        // which will write data we don't care to serialize or decode.
+    }
+
 private:
     typedef SkShader INHERITED;
 };