This extension is used when network connectivity is broken. It's a tiny launcher to open the main Connectivity Diagnostics application.
When the system is offline, the Chrome network pages will include a “Running Connectivity Diagnostics” link. That will run this launcher directly.
See the Connectivity Diagnostics source tree for more details.
This is registered as chrome-extension://idddmepepmjcgiedknnmlbadcokidhoa/.
Visiting chrome-extension://idddmepepmjcgiedknnmlbadcokidhoa/index.html directly will launch the Connectivity Diagnostics app.
This extension is registered directly in the Chromium source. You can see references via code search: https://cs.chromium.org/search/?q=connectivity_diagnostics_launcher&sq=package:chromium&type=cs
This project is mostly maintained in Google3 at //depot/google3/enterprise/deployments/cse/internal/ccd_v1/launcher_extensions/chrome_os/. Changes should be made there and synced back to this copy.
The tests.html and tests.js files are for autotest integration. See the desktopui_ConnectivityDiagnostics test for more details.