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This CL implements transition animation between collapsed state and
expanded state of UnifiedSystemTray.

As seen in the UX prototype, we support state change from touch gesture
and CollapseButton. In case of touch dragging, we should show
intermediate state between transition, depending on how much it's
dragged.

To support this, this CL extends UnifiedSystemTrayView::SetExpanded to
take double instead of bool, and make it render arbitrary intermediate
state. See the design doc for detail.

Video:
http://dr/file/d/1260aGQRNzgN5Y2qH5FhovT7nBbPhqoR8/view?usp=sharing

UX prototype: http://shortn/_EAeqJNHpAg
Design doc: go/cros-qs-restyling (See "Support animation" section)

TEST=manual
BUG=826999

Change-Id: Id010ad58b1fcbf77eac7c37b911fd0d164d7fe57
Reviewed-on: https://chromium-review.googlesource.com/981915Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546766}