[Password Manager] Show export menu item only for a non-empty password list

Exporting an empty password list has no value for the user. Additionally,
exporting while the password list is still loading is undefined behaviour.

For these reasons, we only show the Export Passwords menu item only once
the password list has been read and is non-empty.

Bug: 789561,805534,807975
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ief82b4b46a231402dbaa7251e514c4078f7be31a
Reviewed-on: https://chromium-review.googlesource.com/897486
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534728}

chrome/browser/resources/settings/passwords_and_forms_page/passwords_section.js

@@ -294,10 +294,7 @@ Polymer({
     /** @private */
     showExportPasswords_: {
       type: Boolean,
-      value: function() {
-        return loadTimeData.valueExists('showExportPasswords') &&
-            loadTimeData.getBoolean('showExportPasswords');
-      }
+      computed: 'showExportPasswordsAndReady_(savedPasswords)'
     },
 
     /** @private */
@@ -580,6 +577,16 @@ Polymer({
     return toggleValue ? this.i18n('toggleOn') : this.i18n('toggleOff');
   },
 
+  /**
+   * @private
+   * @param {!Array<!PasswordManager.PasswordUiEntry>} savedPasswords
+   */
+  showExportPasswordsAndReady_: function(savedPasswords) {
+    return loadTimeData.valueExists('showExportPasswords') &&
+        loadTimeData.getBoolean('showExportPasswords') &&
+        savedPasswords.length > 0;
+  },
+
   /**
    * @private
    * @param {boolean} showExportPasswords