fix: don't execute preload scripts for internal <iframe> in <webview> when nodeIntegrationInSubFrames is enabled

[miniak]

Description of Change

Fixes #18429.

The problem is that before the webContents is attached to the internal <iframe> inside of the <webview>, it's treated as a regular frame and the preload scripts are executed there.

Fix by checking v8Util.getHiddenValue(window.frameElement, 'internal') on context initialization, which is set by the <webview> implementation:

electron/lib/renderer/web-view/web-view-impl.ts

Lines 54 to 61 in 50f4977

	createInternalElement () {
	const iframeElement = document.createElement('iframe')
	iframeElement.style.flex = '1 1 auto'
	iframeElement.style.width = '100%'
	iframeElement.style.border = '0'
	v8Util.setHiddenValue(iframeElement, 'internal', this)
	return iframeElement
	}

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Preload scripts for internal <iframe> in <webview> are not longer executed when nodeIntegrationInSubFrames is enabled.

[zcbenz]

The test is failing on Windows CI:

not ok 515 renderer nodeIntegrationInSubFrames with context isolation, webview on should correctly reply to the sub-frames with using event.reply
23533  Error: Timeout of 30000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves. (C:\projects\src\electron\spec\api-subframe-spec.js)
23534      at Test.Runnable._timeoutError (C:\projects\src\electron\spec\node_modules\mocha\lib\runnable.js:440:10)
23535      at C:\projects\src\electron\spec\node_modules\mocha\lib\runnable.js:251:24
23536

[miniak]

@zcbenz I've fixed the tests

[release-clerk]

Release Notes Persisted

Preload scripts for internal <iframe> in <webview> are not longer executed when nodeIntegrationInSubFrames is enabled.

[trop]

I was unable to backport this PR to "5-0-x" cleanly;
you will need to perform this backport manually.

[trop]

I was unable to backport this PR to "6-0-x" cleanly;
you will need to perform this backport manually.

[trop]

A maintainer has manually backported this PR to "6-0-x", please check out #19298

[trop]

A maintainer has manually backported this PR to "5-0-x", please check out #19299

[KochiyaOcean]

https://github.com/electron/electron/blob/master/docs/api/webview-tag.md#nodeintegrationinsubframes
The document still says All your preloads will load for every iframe. Is that behavior no longer occurs after this commit?