runtime: Add cmd line option for changing native host window bounds

Ideally we would like to use ui::DisplayConfigurator for hotplug the monitors
and set the host window bounds automatically based on their mode sizes. That's
not possible though cause Crosswalk embedded solution doesn't fetch ChromeOS
dependencies, which is where ui::DisplayConfigurator situates.

Our solution here therefore is to pass through command-line options the native
bounds, using --xwalk-host-window-bounds=800x600 (or the appropriated monitor
resolution).