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This CL also adds unit tests for the collapse/expand behaviour for the message
center.

TEST=manual + new unit tests
BUG=1019405

Change-Id: I1189549e41e6f11f2d513c4d041de6e5ac108caf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1888187
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711131}