fix: UtilityProcess.fork crash before app ready

[codebytere]

Description of Change

Fixes #41396.
Refs #40017

Fixes an issue where calling UtilityProcess.fork prior to the app ready event would cause a crash. This happened because #40017 leveraged SystemNetworkContextManager, which prior to this wasn't created until after the app ready event. This PR updates our SystemNetworkContextManager logic to more closely reflect upstream, where a call to SystemNetworkContextManager::GetInstance() creates a new instance via ElectronBrowserClient::OnNetworkServiceCreated().

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Fixed an issue where calling UtilityProcess.fork prior to the app ready event would cause a crash.

[nornagon]

seems fine to me!

[deepak1556]

Is the network service fine to be created before IO threads are setup ? We don't allow net module in the main process before app ready for related reasons. I wonder if a better fix would be follow the same pattern in utility process, that is

1. Throw on net module usage before app ready from utility process scripts
2. Lazily initialize network context details in the utility process after app is ready.