Revert "cros: Rename window_selectorXX to overviewXX."

This reverts commit 64eca96f5a2f48ff47a28d38762dc5d76f02baa3.

Reason for revert: Speculative; hard to see how revert affects
but many tablet_mode/overview/splitview tests started to fail
after around this CL:

https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20Chromium%20OS%20ASan%20LSan%20Tests%20%281%29/31172
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/linux-chromeos-dbg/10410

Original change's description:
> cros: Rename window_selectorXX to overviewXX.
> 
> Renames window_selector.cc -> overview_session.cc as overview.cc
> sounded strange.
> 
> Renamed so files scoped_xx_overview_yy to scoped_overview_xx_yy to make
> them match.
> 
> Test: ash_unittests
> Bug: 914147
> Change-Id: Ibc02f88d1ba1b3031bf787b59cc67eb01bf0daa3
> Reviewed-on: https://chromium-review.googlesource.com/c/1419133
> Reviewed-by: Scott Violet <sky@chromium.org>
> Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
> Commit-Queue: Sammie Quon <sammiequon@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#624842}

TBR=sky@chromium.org,oshima@chromium.org,sammiequon@chromium.org

Change-Id: I4a5c71f2e809377229d33873826b633657b4ec54
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 914147
Reviewed-on: https://chromium-review.googlesource.com/c/1429299
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625052}
78 files changed