chrome/common/extensions/docs/templates/private/intro_tables/windows_permissions.html

The <code>chrome.windows</code> API can be used
 without declaring any permission.
 However, the <code>"tabs"</code> permission is
 required in order to populate the
 <code><a href="tabs.html#property-Tab-url">url</a></code>,
 <code><a href="tabs.html#property-Tab-title">title</a></code>, and
 <code><a href="tabs.html#property-Tab-favIconUrl">favIconUrl</a></code> properties of
 <code><a href="tabs.html#type-Tab">Tab</a></code> objects.