wm: Simplify DragWindowController and usage thereof

As a first step toward fixing Issue 965665, the present CL just
simplifies the relevant code.

Test: manual
Bug: 965665
Change-Id: I371a293e8ace81e2b4e325a4e6c36b576e0a46bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623130
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661977}
4 files changed