fix: empty result of webContents.getUserAgent()

[codebytere]

Description of Change

Closes #35046.

Refs #34481 - this change made it such that we were no longer initializing the user agent and so it would always be empty when calling webContents.getUserAgent() unless it had been previously set by webContents.setUserAgent(agent). This fixes that, and incidentally removes the need to have a separate GetUserAgentOverride.

Confirmed not to regress #30201

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are 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 webContents.getUserAgent() incorrectly returning an empty string unless previously set.

[JCMais]

This seems to be affecting the latest v19 patch release too, so a backport may be needed

[miniak]

@codebytere why not 17-x-y? The previous fix, which regressed this was backported to 17-x-y

[codebytere]

@miniak 17 is security fixes only

[miniak]

@codebytere theoretically yes, but that's not what's been done if you check the commit history

[release-clerk]

Release Notes Persisted

Fixed webContents.getUserAgent() incorrectly returning an empty string unless previously set.

[trop]

I have automatically backported this PR to "18-x-y", please check out #35130

[trop]

I have automatically backported this PR to "20-x-y", please check out #35131

[trop]

I have automatically backported this PR to "19-x-y", please check out #35132

[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 #35151

[KrisanaSangduen]

ฉันไม่มีความรู้ความสามารถในการใช้งาน ฉันอยากขอความร่วมมือให้ทุกฝ่ายช่วยประสานงาน ส่งข้อมูลส่วนตัวที่เกี่ยวกับเนื้อหาในการเปิดเผยข้อมูลส่วนตัว และช่วยประชาสัมพันธ์ต่อสื่อสาธารณะที่เหมาะสม และให้เงินทุนสนับสนุนก่อนที่ฉันจะเข้าร่มงานและฝึกทักษะการเนียนรู้ในอนาคตข้างหน้า