GitLab

* Add a <select> element to the dialog allowing the user to choose a category, or "other".
* Disable the OK button and hide the reset options until a category is selected.
* Clean up some duplicated code now that this dialog depends on base.js.

BUG=b/20950552

Review URL: https://codereview.chromium.org/1131153003

Cr-Commit-Position: refs/heads/master@{#329784}