[Extensions Click-to-Script] Bring up dialog on 'specific sites' selection

When the user selects "On specific sites" from the dropdown in the
chrome://extensions page, automatically bring up the dialog to add an
allowed site. This makes it clear to the user that they need to specify
a site for this setting to work, and also fixes an issue where
selecting "On specific sites" and then refreshing the page (or having
the data model otherwise update) reset the value to "on click".

Bug: 884222

Change-Id: Ie89757f7fba2409774e5fcc2f4a8028aff31f7ba
Reviewed-on: https://chromium-review.googlesource.com/1226017
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592118}
5 files changed