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This change is a result of our discussions with the privacy folks.
The old behavior was to center the disconnect window along the
x-axis and dock it just above the taskbar.  The change is to now
position the dialog near the center of the y-axis as well to
maximize the chances of a local user seeing the disconnect window
before it auto-hides.

This change also fixes a bug where I was setting the position after
animating the window so it would appear to jump when being reshown.

BUG=882926

Change-Id: I3e923937f10f24c3e719f311380a98b77644e866
Reviewed-on: https://chromium-review.googlesource.com/1220029Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590434}