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The shelf widget delegate view, which also acts as a shelf widget's
contents view, contains two views - login shelf view, and the drag
handle.

Currently it uses fill layout manager for layout, which makes sense for
login shelf view, but not so much for the drag handle. To avoid drag
handle from getting resized to the whole shelf view bounds, the shelf
widget resets the delegate view layout manager when session state
changes. This works because login shelf and drag handle are mutually
exclusive. With new final first user run oobe screen mocks, which may
have swipe from shelf gesture enabled, login shelf may require drag
handle, which now break with fill layout manager.

This CL changes shelf widget delegate view to manage its own layout - it
sets the login shelf view to full shelf bounds, and centers the drag
handle within the shelf bounds. Also, it makes relationship between
delegate view and login shelf view more explicit, as the login view is
now added to delegate view using explicit AddLoginShelfView method,
instead of using GetContentsView()->AddChildView from shelf widget.

BUG=976949

Change-Id: Ic5a26c4ab657194c4cf4dfc5a4473eca4807146e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2071289
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744353}