fix: don't execute preload scripts for internal <iframe> in <webview> (#19260)

spec/fixtures/sub-frames/webview-iframe-preload.js

@@ -0,0 +1,14 @@
+const { ipcRenderer } = require('electron')
+
+if (process.isMainFrame) {
+  window.addEventListener('DOMContentLoaded', () => {
+    const webview = document.createElement('webview')
+    webview.src = 'about:blank'
+    webview.addEventListener('did-finish-load', () => {
+      ipcRenderer.send('webview-loaded')
+    }, { once: true })
+    document.body.appendChild(webview)
+  })
+} else {
+  ipcRenderer.send('preload-in-frame')
+}