A plugin or a site theme (gerrit-theme.html) may register a custom
Element to be inserted into this endpoint.
``` js
Gerrit.install(plugin => {
plugin.registerCustomComponent(
'header-browse-source', 'gt-header-source', {replace: true});
});
```
``` html
<dom-module id="gt-header-source">
<template>
<a href="http://somecode.com/">Repositories</a>
</template>
<script>
Polymer({is: 'gt-header-source'});
</script>
</dom-module>
````
Feature: Issue 7120
Change-Id: Ib4b133de755c16ae32e464e7c4352310f29ef5b0
f3407cfb98