web-view: stop touching the background color

This was added to improve dark mode by avoiding flashes of light background before loading dark pages. Unfortunately it doesn't work. The call to reset the background color in LOAD_COMMITTED stage is racy: it may happen after the website has already set its own background color, improperly overriding a dark background color to white. This is surely worse than the flash of light that this code is trying to fix. And there is nowhere else we can plausibly reset the background color, so the best we can do is give up. This will need to be fixed in WebKit.