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_NET_WM_STATE holds the window state which may be eg: maximized, fullscreen,
minimized, etc.  In shutdown, Chrome queries this state so that it can restore
browser windows to the correct state on the next launch.  The problem is that
Chrome queries this state (hundreds of times, in fact) after the window has
already gone away.  The EWMH spec requires window managers to delete the
_NET_WM_STATE property when a window is unmapped [1], so the state that Chrome
was getting was invalid.

This CL saves the state when a window becomes unmapped, and includes some small
cleanups.

[1] https://standards.freedesktop.org/wm-spec/wm-spec-1.3.html#idm140130317598336

BUG=882258
R=sky

Change-Id: Iacc43563cd54ade77ac83a580ed24434b6802c91
Reviewed-on: https://chromium-review.googlesource.com/1226645
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592570}