Made cr-dialog-frame not focused by Tab key.

In the situation of pressing the Tab key repeatedly, when cr-dialog-frame has
focus, it looks like focus is disappeared.  This CL change the tabIndex assigned
to cr-dialog-frame from 0 to -1. Elements that have negative tabIndex can be
focused but are not traversed by Tab key.

BUG=239306
TEST=manually

Review URL: https://chromiumcodereview.appspot.com/16123002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@205482 0039d316-1c4b-4281-b951-d872f2087c98

ui/webui/resources/js/cr/ui/dialogs.js

@@ -47,7 +47,9 @@ cr.define('cr.ui.dialogs', function() {
 
     this.frame_ = doc.createElement('div');
     this.frame_.className = 'cr-dialog-frame';
-    this.frame_.tabIndex = 0;
+    // Elements that have negative tabIndex can be focused but are not traversed
+    // by Tab key.
+    this.frame_.tabIndex = -1;
     this.container_.appendChild(this.frame_);
 
     this.title_ = doc.createElement('div');