BrowserWindow extension API is not available on ready since 9

[ChALkeR]

Preflight Checklist

• I have read the Contributing Guidelines for this project.
• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for an issue that matches the one I want to file, without success.
  There were some similar, e.g. TypeError: BrowserWindow.addDevToolsExtension is not a function #22117, but my issue is not reproducible on 8.x.

Issue Details

• Electron Version:
  • 9.0.0
• Operating System:
  • Linux xps 5.6.11-arch1-1 #1 SMP PREEMPT Wed, 06 May 2020 17:32:37 +0000 x86_64 GNU/Linux (Arch Linux Current)
• Last Known Working Electron version:
  • 8.3.0

Expected Behavior

Extension APIs should be available on BrowserWindow on app ready event, as documented.

• https://www.electronjs.org/docs/tutorial/devtools-extension#how-to-load-a-devtools-extension
• https://github.com/electron/electron/blob/v9.0.0/docs/tutorial/devtools-extension.md#how-to-load-a-devtools-extension

Actual Behavior

Extension APIs are not available on app ready event but become available after a delay.

To Reproduce

const { BrowserWindow, app } = require('electron')
console.log(`Electron: ${process.versions.electron}`)
app.on('ready', async () => {
  console.log(`ready: ${Object.keys(BrowserWindow)}`)
  setImmediate(() => console.log(`timeout: ${Object.keys(BrowserWindow)}`))
})

Results:

Electron: 8.3.0
ready: fromId,getAllWindows,getFocusedWindow,fromWebContents,fromBrowserView,addExtension,removeExtension,getExtensions,addDevToolsExtension,removeDevToolsExtension,getDevToolsExtensions
timeout: fromId,getAllWindows,getFocusedWindow,fromWebContents,fromBrowserView,addExtension,removeExtension,getExtensions,addDevToolsExtension,removeDevToolsExtension,getDevToolsExtensions

Electron: 9.0.0
ready: fromId,getAllWindows,getFocusedWindow,fromWebContents,fromBrowserView
timeout: fromId,getAllWindows,getFocusedWindow,fromWebContents,fromBrowserView,addExtension,removeExtension,getExtensions,addDevToolsExtension,removeDevToolsExtension,getDevToolsExtensions

[ChALkeR]

Also the docs should probably be updated to point to session.loadExtension instead.

[ChALkeR]

To anyone reading this: this affects just the compatibility wrapper.

The correct "work-around" is not to wrap this call in setImmediate, but to use the new API: https://github.com/electron/electron/blob/master/docs/api/session.md#sesloadextensionpath

[ChALkeR]

Let's close this as those were removed in main.
Refs: #26696