Add undefined check for computed properties

Same as obervers, in polymer 2 a computed property will run when
at least one dependencies is defined. To match the behavior in v1,
return undefined if not all dependencies are defined.

Bug: Issue 10723
Change-Id: Iaa294d5bc392c6f3970cdbb59213a9d2913e6f64

polygerrit-ui/app/elements/change/gr-file-list-header/gr-file-list-header.js

@@ -133,6 +133,11 @@
     },
 
     _computeDescriptionReadOnly(loggedIn, change, account) {
+      // Polymer 2: check for undefined
+      if ([loggedIn, change, account].some(arg => arg === undefined)) {
+        return undefined;
+      }
+
       return !(loggedIn && (account._account_id === change.owner._account_id));
     },