Support modal windows

Support modal windows

* New ContainerParams
 -  default_parent to specify the default parent when no
    transient parent is specified.
 - modal_container_priority now controls the modal container
   used for a modal window created for the given container.
 - if not specified, it will fallback downwards to find one.
 - if window is specified as always_on_top, it will use top
   most modal container.

* Changed network selector/shutdown dialog to use the new API.

* Other change:
 Separated test windows to athena/test/base/test_windows.h

BUG=410499
TBR=sky@chromium.org,reed@chromium.org
TEST=coverd by unit tests

Committed: https://crrev.com/4903fd36b2b36f00efeb1b7bba81b7de6e9457a3
Cr-Commit-Position: refs/heads/master@{#300192}

Review URL: https://codereview.chromium.org/662763002

Cr-Commit-Position: refs/heads/master@{#300288}