Bug 1861445 - Add the runtime.onPerformanceWarning WebExtension event…

… r=zombie

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 w3c/webextensions#456

Differential Revision: https://phabricator.services.mozilla.com/D194708

UltraBlame original commit: 199b3c24b8b43b8053831d7c287bbe83e43037c4

browser/components/extensions/test/browser/browser.toml

@@ -2025,6 +2025,13 @@ js
 ]
 [
 "
+browser_ext_runtime_onPerformanceWarning
+.
+js
+"
+]
+[
+"
 browser_ext_runtime_openOptionsPage
 .
 js