Clean up chrome/browser/ui/views/tabs/window_finder

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/+/1645178
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666607}

• ash/BUILD.gn[diff]
• ash/public/cpp/window_finder.h[Renamed from ash/wm/window_finder.h - diff]
• ash/wm/window_finder.cc[diff]
• ash/wm/window_finder_unittest.cc[diff]
• chrome/browser/ui/BUILD.gn[diff]
• chrome/browser/ui/views/tabs/DEPS[diff]
• chrome/browser/ui/views/tabs/tab_drag_controller.cc[diff]
• chrome/browser/ui/views/tabs/window_finder.cc[Deleted - diff]
• chrome/browser/ui/views/tabs/window_finder.h[diff]
• chrome/browser/ui/views/tabs/window_finder_chromeos.cc[diff]