Fix "A Polymer() declaration cannot use 'const'"

This error is one of many that shows up when setting
`compilation_mode="SIMPLE"` and is fixed in preparation:

ERROR - A Polymer() declaration cannot use 'const'.
  const GrReporting = Polymer({
                      ^
  ProTip: "JSC_POLYMER_INVALID_DECLARATION" can be added to the `suppress` attribute of:
  //polygerrit-ui/app:polygerrit_ui_closure_lib

Change-Id: I2f43b937bc9508270f56b81e955dfebfcd4a2ac0
diff --git a/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js b/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js
index 3e886d5..36126e4 100644
--- a/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js
+++ b/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js
@@ -135,7 +135,9 @@
 
   GrJankDetector.start();
 
-  const GrReporting = Polymer({
+  // The Polymer pass of JSCompiler requires this to be reassignable
+  // eslint-disable-next-line prefer-const
+  let GrReporting = Polymer({
     is: 'gr-reporting',
 
     properties: {