Some browser extensions, like React DevTools, send messages to the content area.
Since our beforeunload event test listens for all messages, it used to catch
those as well, failing the test.
Add a `source` field to the payload JSON and check for it before treating the
message as coming from our own test to make sure the test passes even with such
browser extensions installed.
Closes gh-5478
(cherry picked from commit 399a78ee9f)
