fix: BrowserWindow transparency not working

[codebytere]

Description of Change

Closes #30082.

Fixes an issue where BrowserWindows would not properly honor transparency: true or a backgroundColor being set in their constructor options. This happened as a result of this CL rolled into Electron via #29751. The new method SetPageBaseBackgroundColor set the default content background color - i.e, the color that would appear behind the content unless the content overrode it. What then happened was that when we changed our logic to call this method, it was eventually overridden by Blink, and so would show as transparent or with the supplied color for a flash and then revert to white. We fix this by directly calling the background color method in RenderWidgetHostViewBase.

Tested with https://gist.github.com/b52f026c763057f97f745fade82758fd.

Checklist

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

Release Notes

Notes: Fixed an issue where BrowserWindows would not properly honor transparency: true or a backgroundColor being set in their constructor options.

[nornagon]

This seems to break fairly frequently... is there some way we could test this? With desktopCapturer perhaps? 😬

[codebytere]

Failure not germane.

[release-clerk]

Release Notes Persisted

Fixed an issue where BrowserWindows would not properly honor transparency: true or a backgroundColor being set in their constructor options.

[trop]

I have automatically backported this PR to "14-x-y", please check out #30159

[gr0uch]

This appears to still be broken in latest v14 beta and v15 alpha versions :(

Works on current v13 stable though.