fix: <webview> background transparency

[miniak]

Description of Change

Fixes #31686 regressed by #30777

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 <webview> background transparency regression.

[miniak]

I am hitting this DCHECK when running this code change with a testing build:

FATAL:render_widget_host_view_child_frame.cc(311)] Check failed: SkColorGetA(color) == SK_AlphaOPAQUE || SkColorGetA(color) == SK_AlphaTRANSPARENT.

RenderWidgetHostViewChildFrame::UpdateBackgroundColor()

Everything works properly however when I comment out the DCHECK or run the release build.
The color is actually #ff000080, which I am setting in CSS in the Fiddle repro in the bug-report.

[release-clerk]

Release Notes Persisted

Fixed <webview> background transparency regression.

[trop]

I have automatically backported this PR to "16-x-y", please check out #31728

[TanninOne]

@miniak This bug also exists in 15-x-y (and 14-x-y afaik) could this be backported to those versions as well?

[zcbenz]

/trop run backport

[trop]

The backport process for this PR has been manually initiated - here we go! :D

[trop]

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

[trop]

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