fix: css transparent background being lost

[codebytere]

Description of Change

Closes #31862.

Background colors are a bit of a mess right now in Chromium. They're working on cleaning things up, but in the process have made it harder to determine background color precedence as well as how we can override certain defaults. If we in Electron set const mainWindow = new BrowserWindow({ frame: false, transparent: true, vibrancy:'under-window' }) and then set the CSS background to green the initial content background color gets set to green and at no point after that can be set back to transparent via CSS. This problem was exascerbated by CL:2695928 but happened before that too - before that setting background: transparent would work for about ~4 seconds before being overridden by the initial color. This is only an issue for transparency, as Chromium does not have the same concept of transparency as we do and disfavors or disallows it being set in various places along background color codepaths.

@VerteDinde and I paired on this for a few hours last week and found that above Google doc as well as a content::WebContentsObserver we could override to address this issue in the short term. As such, this PR aims to fix this issue by overriding OnBackgroundColorChanged method to correct transparency when the background color is changed via CSS.

There is most likely a better solution for this, but the chief goal of this PR is to unblock the 17-x-y release which is why i'm putting it up in its current state.

Tested with https://gist.github.com/b23b66ba4e246effc5d29cecb0a40e26

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• 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 an issue where frameless vibrant windows would not show transparency correctly on macOS.

[miniak]

We are running into a related issue in Teams, which unfortunately does not have a simple repro that I can share. I am discussing it 1:1 with @codebytere

[release-clerk]

Release Notes Persisted

Fixed an issue where frameless vibrant windows would not show transparency correctly on macOS.

[trop]

I was unable to backport this PR to "14-x-y" cleanly;
you will need to perform this backport manually.

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I was unable to backport this PR to "15-x-y" cleanly;
you will need to perform this backport manually.

[trop]

I have automatically backported this PR to "16-x-y", please check out #32648

[trop]

I have automatically backported this PR to "17-x-y", please check out #32649

[trop]

@VerteDinde has manually backported this PR to "14-x-y", please check out #33507