The new migration dialog shows an icon for each browser that can be imported from. The simplest solution to display these is to include the branding in the omni.ja as graphic assets.

This bug tracks the work to both:

1. Retrieve and package the branding assets to browser/components/migration/content/brands
2. Update the MigratorBase base class to include a new getter that returns a chrome:// URI string to the appropriate branding asset

Here's how to approach this bug:

1. Download and unzip the folder locally. Move the files into a new folder, browser/components/migration/content/brands, and use hg add to add them.
2. Update the migration component's jar.mn file to make sure we "package" the new images when we build Firefox. You can do that by adding this line:

	content/browser/migration/brands (content/brands/*.png)

3. Then, let's add a new static key to the MigratorBase class to be overridden by child classes, like this one. Let's call it brandImage or something, and have it return "chrome://global/skin/icons/defaultFavicon.svg" by default.
4. Update each subclass of MigratorBase to implement that static method, and return a string URL pointing to the appropriate image file. For example, for the SafariProfileMigrator, I think this would be "chrome://browser/content/migration/brands/safari.png".

Backed out for causing mochitest failures on browser_all_files_referenced.js

Backout link

Push with failures

Failure log

Looks like we also missed an icon for Vivaldi, so I'm attaching that here.

https://hg.mozilla.org/mozilla-central/rev/41710d48be48