runtime: Add aura-only native support

This CL brings aura-only native support for the browser runtime, where views
toolkit dependency is opt out. This is particularly useful for embedded and
limited systems, where the runtime is used always as a fullscreen application
to the host window system. This is aimed to work with Ozone DRI or GBM
implementations.

Testing was performed on Ubuntu Linux 14.04 platform.

TEST= $ python xwalk/gyp_xwalk -Dchromeos=0 -Duse_ozone=1 -Dembedded=1 -Duse_udev=1 -Dozone_platform_gbm=1 -Dozone_platform_wayland=0 -Ddisable_nacl=1
      $ ninja -j16 -C out/Release xwalk
      $ ./out/Release/xwalk --ozone-platform=gbm http://hexgl.bkcore.com/play/
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=800,600 (or whatever is the
appropriated monitor resolution). One would also just link this newly option
with sysfs output like --xwalk-host-window-bounds=`cat /sys/class/graphics/fb0/virtual_size`