Get more details about error in metrics

Adding crossorigin attribute to main js will get more error details
instead of 'script error'.

Change-Id: Id959fdcc1337cf379eb199fb6a2d04252db0ba51

resources/com/google/gerrit/httpd/raw/PolyGerritIndexHtml.soy

@@ -171,5 +171,5 @@
 
   // Load gr-app.js after <gr-app ...> tag because gr-router expects that
   // <gr-app ...> already exists in the document when script is executed.
-  <script src="{$staticResourcePath}/elements/gr-app.js"></script>{\n}
+  <script src="{$staticResourcePath}/elements/gr-app.js" crossorigin="anonymous"></script>{\n}
 {/template}