fix: make preload calculation identical between sandbox & non-sandboxed

[nornagon]

Description of Change

This changes non-sandboxed renderers to load preload scripts the same way that
sandboxed renderers do: by sending a synchronous IPC to the main process to ask
for what preloads should be used.

This does change behavior in some cases. In particular, when running
window.open('about:blank') from a window with a preload and
nodeIntegrationInSubFrames: true, previously the child window would inherit
the parent window's preloads, due to the fact that about:blank child windows
don't go through a browser-side navigation in Chromium (see here
for further details).

Checklist

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

Release Notes

Notes: Fixed child about:blank windows from inheriting their parents' preload scripts when sandbox: false and nodeIntegrationInSubFrames: true.

[release-clerk]

Release Notes Persisted

Fixed child about:blank windows from inheriting their parents' preload scripts when sandbox: false and nodeIntegrationInSubFrames: true.

[trop]

I have automatically backported this PR to "20-x-y", please check out #34585