fix: ensure child window transparency works (#28112)

* fix: ensure child window transparency works

Windows opened via window.open and intecepted via setWindowOpenHandler
or the `new-window` event should (a) have the correct background color
and (b) that background color should be transparent if specified.

The changes in api_web_contents fix (a) and the changes in
web_contents_preferences fix (b).

Notes: Child windows with specified background colors or transpency now
work as intended

* fix: set background_color in blink prefs apply logic

* chore: update for PR comments

Co-authored-by: Samuel Attard <sattard@slack-corp.com>

lib/browser/api/web-contents.ts

@@ -592,6 +592,7 @@ WebContents.prototype._init = function () {
           // it's technically a BrowserWindowConstructorOptions option because
           // we need to access it in the renderer at init time.
           backgroundColor: windowOpenOverriddenOptions.backgroundColor,
+          transparent: windowOpenOverriddenOptions.transparent,
           ...windowOpenOverriddenOptions.webPreferences
         } : undefined;
         this._setNextChildWebPreferences(