feat: sandbox renderer processes for cross-origin frames

[miniak]

Description of Change

Cross-origin frames, which have a separate renderer process due to site isolation do not have their own webContents and therefore use the webPreferences of the main frame. When nodeIntegrationInSubFrames is not enabled, there is no reason not to sandbox the renderer process as we are not executing any internal JavaScript code or initializing node, which would be impacted by the sandbox.

Follow up to #15821

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant documentation is 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: Renderer processes hosting cross-origin frames are now sandboxed unless the parent BrowserWindow enables nodeIntegrationInSubFrames.

[MarshallOfSound]

Why? What's the potential impact? Why do we need to explicitly do this?

[miniak]

@MarshallOfSound I've added a bit more text to the description

Why?

improved security, cross-origin iframes are sandboxed even if the main page is not

What's the potential impact?

there should be no negative effects

Why do we need to explicitly do this?

because there is no way to explicitly enable sandbox on iframes

[miniak]

@MarshallOfSound can you please check again?

[release-clerk]

Release Notes Persisted

Renderer processes hosting cross-origin frames are now sandboxed unless the parent BrowserWindow enables nodeIntegrationInSubFrames.