wm: Remove overview references in CaptionContainerView.

The present CL is based on Sammie Quon's [2], with its goal for
CaptionContainerView to have nothing specific to overview. I am helping
now because my [1] got in the way, adding more stuff specific to
overview in CaptionContainerView while [2] was WIP. Code changes copied
from [2] should be attributed to Sammie Quon rather than to me.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1570218
[2] https://chromium-review.googlesource.com/c/chromium/src/+/1575562

Test: manual
Bug: 914948, 934977
Change-Id: I6cc04c6a8f9126dba327ad45307616115df22a23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577624
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Reviewed-by: Mitsuru Oshima (Slow 4/22-26) <oshima@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654072}
8 files changed