Build NativeWidgetMojo.

We currently have a NativeWidgetViewManager which derives from
NativeWidgetAura. This means that to use Views, you must use Aura. This
minimal implementation of the NativeWidgetPrivate interface can output
to a compositor, and handles mouse input events.

In the long term, actually replacing NativeWidgetViewManager means
removing NativeWidgetAura because NativeWidget has a static interface
which we also probably want to remove.

This also changes the frames in the wm_flow demo so that they use
NativeWidgetMojo.

BUG=439189
R=sky@chromium.org

Review URL: https://codereview.chromium.org/797453002