Bug 32308: use direct browser sizing for letterboxing.

This MR switches letterboxing from using margins to shrink the browser size indirectly to setting the quantized dimensions directly onto the XUL browser element.

Among the benefits of this approach:

1. By relying on the browserStack's grid layout to center the browser automatically, we avoid most of the jittering (fixing #32308 (closed))
2. By not having the margins bouncing the browser bounds around, we prevent most of the transient size inconsistencies (and should other bugs like #41515 (closed))
3. We can completely remove the inter-process communication previously needed in order to listen for resize events in the content process and notify the parent process which adjusted the browser's margins accordingly

As a result, we're removing a significant amount of code, including the RFHHelper parent/child actors (@richard will love this 😸), and simplifying the calculations to the bare minimum.