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This CL completely rewrites MessagePopupCollection which manages popup
notifications. The requirement for the class was changed over time, and
that made the class fragile. Also, there were too many internal states.

For simplicity and full test coverage, MessagePopupCollection manages
all animation states, and only single animation runs at one time.
(This is different from ToastContentsView. The class corresponds to
MessagePopupView in this CL, and it has fewer responsibilities.)

Also this CL has new Chrome OS popup animation in mind. go/qs-proto

As a further refactoring, merging of PopupAlignmentDelegate to
MessagePopupCollection is planned.

Design doc: go/chrome-popup-refactoring

BUG=863366
TEST=MessagePopupCollectionTest

Change-Id: I77c908423c03869d4ac3fa04133f41b73877bd3b
Reviewed-on: https://chromium-review.googlesource.com/1132720Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579349}