fix: BrowserView setBackgroundColor needs two calls

[codebytere]

Description of Change

I determined in testing that #31746 regressed #29778.

It remains the case that we want to call SetPageBaseBackgroundColor on the BrowserView's webContents, but we don't want to do this at the expense of calling SetBackgroundColor on the RenderWidgetHostView. We need to do both calls - similar to what's done here:

electron/shell/browser/api/electron_api_web_contents.cc

Lines 1462 to 1463 in 7e328c4

	web_contents()->SetPageBaseBackgroundColor(color);
	rwhv->SetBackgroundColor(color.value_or(SK_ColorWHITE));

Tested with

• https://gist.github.com/kevinlinv/2de60478ffec6a2f18f1ec0fc6f5add9 to ensure default transparency
• https://gist.github.com/712a2fde2757736c9dbf6c3e7e0aa18b

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 a potential issue when setting backgroundColor on BrowserViews.

[release-clerk]

Release Notes Persisted

Fixed a potential issue when setting backgroundColor on BrowserViews.

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I was unable to backport this PR to "13-x-y" cleanly;
you will need to perform this backport manually.

[trop]

I have automatically backported this PR to "14-x-y", please check out #31945

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I have automatically backported this PR to "15-x-y", please check out #31946

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I have automatically backported this PR to "16-x-y", please check out #31947

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I have automatically backported this PR to "17-x-y", please check out #31948

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@VerteDinde has manually backported this PR to "13-x-y", please check out #32704