fix: make navigator.userAgentData non-empty

[nornagon]

Description of Change

Fixes #30201.

Previously, Electron was always overriding the user-agent string, but not the
user-agent metadata. This resulted in navigator.userAgentData being empty,
and Sec-CH-UA headers not being sent.

With this change, the user-agent data is set correctly & should be sent with
Sec-CH-UA and should be visible in navigator.userAgentData. One notable
fact is that the brands field does not contain Electron, only Chromium.
I'm not sure if this is something we want to change.

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 missing Sec-CH-UA headers and empty navigator.userAgentData.

[PalmerAL]

One notable fact is that the brands field does not contain Electron, only Chromium. I'm not sure if this is something we want to change.

This is possibly a preemptive comment, but for my use case, I want the UA header to be identical to Chromium's (and therefore not contain Electron). If you do change this, could you add a way to disable it (or more generally, an API like session.setUserAgent to set the userAgentData, which would be even better)?

[release-clerk]

Release Notes Persisted

Fixed missing Sec-CH-UA headers and empty navigator.userAgentData.

[trop]

I have automatically backported this PR to "20-x-y", please check out #34524

[yellow1234]

I did test with new BrowserWindow/loadURL to:
https://www.bing.com/search?q=car

After this merge the low entropy headers
sec-ch-ua
sec-ch-ua-mobile
sec-ch-ua-platform

Are working, but only from the second request to origin (after response with "Accept-CH" header)
if you test in chrome incognito, you will see that its send those headers from the first request

Also, the high entropy headers aren't working at all
sec-ch-ua-arch
sec-ch-ua-full-version
sec-ch-ua-full-version-list
sec-ch-ua-platform-version
sec-ch-ua-bitness
sec-ch-ua-wow64

Electron never sending those, even after response with "Accept-CH" header
like this:
Accept-CH: Sec-CH-UA-Bitness, Sec-CH-UA-Arch, Sec-CH-UA-Full-Version, Sec-CH-UA-Mobile, Sec-CH-UA-Model, Sec-CH-UA-Platform-Version, Sec-CH-UA-Platform, Sec-CH-UA, UA-Bitness, UA-Arch, UA-Full-Version, UA-Mobile, UA-Model, UA-Platform-Version, UA-Platform, UA

[nornagon]

@yellow1234 please file a new bug with a test case, thanks!

[miniak]

/trop run backport-to 19-x-y

[trop]

The backport process for this PR has been manually initiated - sending your PR to 19-x-y!

[miniak]

/trop run backport-to 18-x-y

[trop]

The backport process for this PR has been manually initiated - sending your PR to 18-x-y!

[trop]

I have automatically backported this PR to "19-x-y", please check out #34757

[trop]

I have automatically backported this PR to "18-x-y", please check out #34758

[miniak]

/trop run backport-to 17-x-y

[trop]

The backport process for this PR has been manually initiated - sending your PR to 17-x-y!

[trop]

I have automatically backported this PR to "17-x-y", please check out #34760