Display "Find Owners" button after switching pages.

When switching PolyGerrit pages, the "Find Owners" button could be hidden.
Set it to non-hidden when a page is shown again.

Change-Id: Ie08fb0911b61dde07d396608f887cc461ffefb19

src/main/resources/static/find-owners.js

diff --git a/src/main/resources/static/find-owners.js b/src/main/resources/static/find-owners.js
index a0811ce..24a8554 100644
--- a/src/main/resources/static/find-owners.js
+++ b/src/main/resources/static/find-owners.js
@@ -505,16 +505,17 @@
     }
     return true; // Okay to submit.
   }
-  var hasPolyGerritButton = false;
+  var actionKey = null;
   function onShowChangePolyGerrit(change, revision) {
     var changeActions = self.changeActions();
     // Hide previous 'Find Owners' button under 'MORE'.
     changeActions.setActionHidden('revision', 'find-owners~find-owners', true);
-    if (!hasPolyGerritButton) {
-      var key = changeActions.add('revision', 'Find Owners');
-      changeActions.addTapListener(key,
+    if (!!actionKey) {
+      changeActions.setActionHidden('revision', actionKey, false);
+    } else {
+      actionKey = changeActions.add('revision', 'Find Owners');
+      changeActions.addTapListener(actionKey,
           () => popupFindOwnersPage(null, change, revision, false));
-      hasPolyGerritButton = true;
     }
   }
   function onClick(e) {