fix: BrowserWindow backgroundColor

[samuelmaddock]

Description of Change

fixes #30759

Port of #30777 which targets v14+15. I manually ported this to latest because of the changes to WebPreferences in #30193

The code below would fail to convert a hex string to an SkColor which resulted in the RVHW's background color always being set to transparent.

electron/shell/browser/web_contents_preferences.cc

Line 227 in dd7aeda

web_preferences.Get(options::kBackgroundColor, &background_color_);

cc @codebytere @nornagon

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: none

[samuelmaddock]

@nornagon I found a better approach for this.

To make a WebContents transparent, we only need to set the background color to SK_ColorTRANSPARENT. The transparent override in WebViewImpl::BaseBackgroundColor() isn't strictly necessary.

It also doesn't make sense to enforce transparency of the WebContents based on the options passed to BrowserWindow. The user should be able to change the background color to not be transparent at any time.

With these changes, WebContents transparency via transparent: true is equivalent to backgroundColor: '#00000000' or browserWindow.setBackgroundColor('#00000000').

[release-clerk]

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