fix: emit IPC event in correct context if isolation and sandbox enabled (#16352)

* fix: emit IPC event in correct context if isolation and sandbox enabled

IPC events were not being delivered to renderer processes when both
`contextIsolation` and `sandbox` were enabled. This is because the
`AtomSandboxedRenderFrameObserver` class was incorrectly using the
`MainWorldScriptContext`, rather than conditionally selecting the
context based on if isolation was enabled.

Fixes #11922

spec/fixtures/module/preload-ipc-ping-pong.js

@@ -0,0 +1,9 @@
+const { ipcRenderer } = require('electron')
+
+ipcRenderer.on('ping', function (event, payload) {
+  ipcRenderer.sendTo(event.senderId, 'pong', payload)
+})
+
+ipcRenderer.on('ping-æøåü', function (event, payload) {
+  ipcRenderer.sendTo(event.senderId, 'pong-æøåü', payload)
+})