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We currently explicitly add the keyboard container to the root window
in each test case. We might as well do it in SetUp.

We also clean up the tests a bit. This will be helpful when we remove
the keyboard container.

Bug: 849995
Change-Id: I9c188b215bca62f3fd5fddc5478d898106a05a53
Reviewed-on: https://chromium-review.googlesource.com/1098741Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567434}