[WebPayments] Introduce OverlayWidget & Use it by PaymentHandler

[javifernandez]

A new OverlayWidget class is defined to render the payment handler WebContents instance, provided by the web engine, as a result of the call to the Payment Request API.

The tab that initiates the payment request needs to handle 2 WebContents instances, determining which one is active during the process. Hence, The OverlayWidget is implemented as modal dialog, preventing the original web-content to be updated until the payment request is completed.

The TabsWebContentObserver must implement a new WebContentsObserver method, defined to notify Wolvic that the new payment handler WebContents instance is ready.

This PR depends on the PR #143 in the Chromium backend.

[mshin-wolvic]

LGTM but I'd like to defer to others who take a look with other points of view.

[svillar]

what should we do with #1646 then?

[javifernandez]

what should we do with #1646 then?

This PR is based on #1646 it's basically the same code, with some cleanup and removed logging and comments.

[svillar]

what should we do with #1646 then?

This PR is based on #1646 it's basically the same code, with some cleanup and removed logging and comments.

So should we close the other?

[mshin-wolvic]

what should we do with #1646 then?

This PR is based on #1646 it's basically the same code, with some cleanup and removed logging and comments.

So should we close the other?

FYI, I closed other tickets.

[svillar]

I've done a general review and while the code is clean and easy to understand I have the feeling that it's a bit ad-hoc. Anyway, as I am not totally sure, and provided that it deserves a more in depth review, I'll neither approve nor request changes yet.