Mus Demo: Reorganize functions in mus_demo.cc

In [1], some functions were moved from the MusDemo class to the
MusDemo::WindowTreeData class. Their bodies were however not moved
inside mus_demo.cc (in order to facilitate review). This is now done in
this CL and MusDemo::WindowTreeData member functions are grouped together
at the same location. At the same time, the body of the constructor is
actually moved to a separate Init function so that it can be used by
another constructor in a follow-up CL.

BUG=666958
R=rjkroege@chromium.org

[1] https://codereview.chromium.org/2679213003/

Review-Url: https://codereview.chromium.org/2688123002
Cr-Commit-Position: refs/heads/master@{#449725}
1 file changed