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Since Mash / WS is gone, the window finder class can be significantly
simplified. This CL does this simplification.

Also moves ash/wm/window_finder.h to ash/public/cpp; this file
would be helpful for others.

Bug: none
Test: trybot
Change-Id: I0dc745c3f5b5ae8d09afbea6930e6203dac29e73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1645178Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666607}