Use webcontents ID in isSender check

[colin-grant-work]

What it does

Fixes #11600 according to @kittaakos suggestion.

How to test

1. Run the electron example: (We can see the window 1)
2. Reload the window: (Reload the window 1)
3. Click the New Window in the menu, and it does work.

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1. Open a file and make it dirty.
2. Click to exit the application.
3. See a dialog prompting you for whether you want to exit.
4. Click no
5. The window should remain open.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[kittaakos]

Thank you for the prompt fix!

How to test

1. Run the electron example: (We can see the window 1)
2. Reload the window: (Reload the window 1)
3. Click the New Window in the menu, and it does work.

It does not work for me. I have checked out 4a646eb and built it locally. It also does not work when opening a new window with ⌘+Shift+N. I am investigating what's going on.

steps_001.mp4

[kittaakos]

This change fixes the issue for me. I think the window multiplexer should be removed from the map when the window reloads/closes.

[colin-grant-work]

Since this doesn't seem to have improved anything, I'll close it. @kittaakos, if you'd like to make a PR with your changes, feel free.

[kittaakos]

It fixes the reload error described in #11600 for me.