If the CSS filter was enabled, when the user would navigate to a page
on a different origin, the CSS Warnings from the new page wouldn't
be displayed in the console.
This is related to how we manage the CSS Warnings. Since emitting those
messages is costly, we only do so when the console is opened, if
the user already set the filter, or when they turned it on.
The issue is that it was only done on the main target, and only
when the console would start, or when the user clicked on the css
So with Fission enabled, we could switch to a new target, but we
wouldn't trigger the code that parses the stylesheets of the new
This patch changes how we trigger the CSS warning report mechanism,
by not listening to specific actions, but having a Set of already
handled targets. When it encounters a new target, if the css filters
is enabled, then it will enable the CSS warning messages to be
The browser_webconsole_message_categories was asserting this issue,
and is now fixed (after setting the proper target switching target).