Print Preview Componentization: Fix focus issue

Ensure focus remains in the textfield after the user enters a new value,
even if this changes the validity of the input.

Bug: 843952
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I0fec55a89c10d4b4dcbaff219927ca26fd650594
Reviewed-on: https://chromium-review.googlesource.com/1069254Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560880}

chrome/browser/resources/print_preview/new/settings_behavior.js

@@ -92,8 +92,9 @@ const SettingsBehavior = {
     // is no way for the user to change the value in this case.
     if (!valid)
       assert(setting.available, 'Setting is not available: ' + settingName);
-    if (valid != setting.valid)
-      this.fire('setting-valid-changed', valid);
+    const shouldFireEvent = valid != setting.valid;
     this.set(`settings.${settingName}.valid`, valid);
+    if (shouldFireEvent)
+      this.fire('setting-valid-changed', valid);
   }
 };