For a while now in the chrome developer tools, the tab icons have not been showing. Here is a screenshot of what I mean:
This is what it should look like:
This is by design. They've changed the layout to be more compact.
You can set this back to the old default in the Developer Tools settings:
In the Developer Tools pane, click the gear icon:
Now, tick Show Toolbar Icons.
This option was removed from Chrome in version 30.
You can no longer set toolbar icons, unless one is provided in an unofficial theme for the devtools: http://devthemez.com/themes/chrome-developer-tools?sort=popular