cros: Make WallpaperWindowStateManager work under Mash

Create mojo API and move the tests in 'wallpaper_private_api_unittest.cc'
to //ash, because:

1) They're only testing wallpaper_window_state_manager.

2) We want to remove //ash dependency in chrome wallpaper code.

(IIUC)|MultiUserWindowManagerChromeOS| under //chrome is responsible
for tracking the window ownership under multi-profile session. The
wallpaper_window_state_manager is only checking if the window is
visible. So if we move the tests to //ash, they only need to test that
the minimize/restore functions are no-op for invisible windows. Why
the windows are visible/invisible is beyond its scope.

Bug: 827062
Test: --enable-features=Mash
Change-Id: Ib7573e8911a6979f598f85cbb0b059be1c77b389
Reviewed-on: https://chromium-review.googlesource.com/967885
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Reviewed-by: Oliver Chang <ochang@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549811}
13 files changed