Devtools: Remove extra tab stop from Settings

Settings uses UI.Dialog to show the Dialog. And Settings.SettingsScreen is where the content for Settings are added.
This change removes extra tabIndex in constructor of Settings.SettingsScreen and updates the dialog's tabIndex to -1 from 0

Before: https://imgur.com/1M08W3X
After: https://imgur.com/5FtlcYc

Bug: 963183

Change-Id: I5caa95d2aba4aa0b2433d1f6464b328954cc7165
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715542
Reviewed-by: Yang Guo <[email protected]>
Commit-Queue: Chandani Shrestha <[email protected]>
Cr-Original-Commit-Position: refs/heads/master@{#702080}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 1bf412107ab9ddc5aa4e0e619703ed3838b14066
diff --git a/front_end/settings/SettingsScreen.js b/front_end/settings/SettingsScreen.js
index 0e9db47..728a7b8 100644
--- a/front_end/settings/SettingsScreen.js
+++ b/front_end/settings/SettingsScreen.js
@@ -36,7 +36,6 @@
     super(true);
     this.registerRequiredCSS('settings/settingsScreen.css');
 
-    this.contentElement.tabIndex = 0;
     this.contentElement.classList.add('settings-window-main');
     this.contentElement.classList.add('vbox');
 
@@ -73,6 +72,7 @@
       return;
     }
     const dialog = new UI.Dialog();
+    dialog.contentElement.tabIndex = -1;
     dialog.addCloseButton();
     settingsScreen.show(dialog.contentElement);
     dialog.show();