Viewport meta tag not working in v5.8.0-beta1

Passing --enable-viewport in 5.7 would also enable the viewport meta tag reading from the page to setup the layout and scaling constraints. This doesn't seem to work anymore in 5.8.

`content::WebPreferences::viewport_meta_enabled` is now hardcoded to false unless `OS_ANDROID` is defined.

I tested on desktop but I'm assuming that this is broken on embedded too, attached is an example patch that would fix it for me (unlicensed, public domain). I can't test on embedded with a touch screen easily at the moment.