When an extension's content script is very slow and causes a webpage
to hang noticeably, a warning banner is displayed to the user. It
would be useful to also notify the extension developer when that
happens, so that they can address the issue.
Let's add a new event runtime.onPerformanceWarning that can be
dispatched when the browser needs to warn an extension of runtime
performance issues. For now, let's just dispatch that event when the
slow extension warning banner is displayed to the user.
See also https://github.com/w3c/webextensions/issues/456
Differential Revision: https://phabricator.services.mozilla.com/D194708