Sample for chrome.desktopCapture

I would like to write a sample for the `chrome.desktopCapture` API.

The idea is the following:
1. the user click on the extension icon and the built-in popup is displayed

2. the user choose the source to capture

3. the stream is displayed in a new window

No file or test is named. Start by reviewing the repository's existing samples and the chrome.desktopCapture API entry points. Add a sample covering the stated flow: opening the extension popup, choosing a capture source, and displaying the resulting stream in a new window.

Written by the indexing model from the issue text.