The "Wm" porting layer is being removed, so it can go back to its
original name.

No behavior changes, just a mechanical refactor.

tools/git/move_source_file.py for
    wm_shelf.h to shelf.h
    wm_shelf.cc to shelf.cc
    wm_shelf_observer.h to shelf_observer.h

tools/git/mffr.py for
  WmShelfObserver to ShelfObserver
  WmShelf to Shelf
  wm_shelf to shelf

Fix up a couple call sites like "wm_shelf_container" window.

Replace deprecated GetAlignment() with alignment() because it touches
the same lines as the rest of this change.

BUG=none
TEST=compiles, ash_unittests, chrome unit_tests
R=msw@chromium.org, tsepez@chromium.org
TBR=reveman@chromium.org, xiyuan@chromium.org

Review-Url: https://codereview.chromium.org/2899253002 .
Cr-Commit-Position: refs/heads/master@{#474526}