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UX spec dictates that if not in stylus mode:
- Height should be >= 200dip.
- Height should grow to match contents.
- Height should not shrink until Assistant UI is closed.

Note that now UiElementContainerView needs to always be visible so
that it can fill the remaining space, even when empty of children.

Demo in bug.

Bug: b:79432250
Change-Id: I32372467bb8fff2b74885014b34152ce907be1bf
Reviewed-on: https://chromium-review.googlesource.com/1074373
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562594}