Issue: WebContent may remain present after closing Ladybird.

Reproducing difficulty: Very easy with certain websites

HEAD at testing time: commit 41cc8e7 (HEAD -> master, origin/master, origin/HEAD)
(As as a reference when was this tested)

When it does not happen:

• Doing CTRL+C in the terminal (seems to) always kill spawned processes (WebContent does not become a zombie process)

When it does happen:

• Pressing the X button or doing File -> Quit when closing the browser

Suggested conditions required for the bug to happen:

1. The browser must be busy with something
2. The webpage must be very complex or broken

In order for 1) to happen:

1. Open a webpage
2. Wait for it to load + give it a few seconds to stay idle
3. Start clicking everywhere, spamming F5 etc
4. Quickly close the browser using File -> Quit or the X button
5. Ladybird closes, but WebContent becomes a zombie process

Demonstration:
(Disclaimer: Mojeek is a relatively lightweight page. It is hard to make Ladybird busy with it, but it does work)

In order for 2) to happen:

1. Load something broken, like for example https://duolingo.com or complex like https://adilene.net/
2. Wait like 10-15 seconds (You don't even need to spam anything, just wait)
3. So far 99,999% of the time closing the browser at this stage will result in WebContent becoming a zombie process.

Exaggerated demonstration:

And here is a small comparison:
One browser is closed with File -> Quit and leaves a zombie process, the other is closed using CTRL+C in the terminal and that does not leave any zombie process.