Replace `.src` with `.setAttribute` for gr-lib-loader

Change-Id: I293b9ef0a65ca299f04d465f64bd2766afc9d7f6
diff --git a/polygerrit-ui/app/elements/shared/gr-lib-loader/gr-lib-loader.js b/polygerrit-ui/app/elements/shared/gr-lib-loader/gr-lib-loader.js
index 08e668b..853a299 100644
--- a/polygerrit-ui/app/elements/shared/gr-lib-loader/gr-lib-loader.js
+++ b/polygerrit-ui/app/elements/shared/gr-lib-loader/gr-lib-loader.js
@@ -135,7 +135,7 @@
           return;
         }
 
-        script.src = src;
+        script.setAttribute('src', src);
         script.onload = resolve;
         script.onerror = reject;
         Polymer.dom(document.head).appendChild(script);