AKA magnification / docked magnifier / fullscreen magnifier.

This needs:
1. Actual magnification support (something like ui::Reflector?)
2. IME support for input caret bounds tracking

For (2), the existing ash::MagnificationController assumes there is a single InputMethod object across all displays and windows. It uses an InputMethodObserver to track caret changes. Under mash there is one InputMethod per top-level widget (and per display), similar to other desktop platforms. The magnification code will need to be adapted to track caret changes some other way.

See  issue 548407  for IME notes.

See blocked-on bugs for details.