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- The backdrop used to show immediately even if the
  window-needing-it is still animating, making the animation
  look poor.
- There multiple recursive calls to UpdateBackdrop() constantly
  all the time, many of them are not necessary, and can be handled
  only by laying out the backdrop if needed.
- Even the browser's status bubble used to cause multiple called
  to UpdateBackdrop() as the user moves their mouse hovering over
  links, which causes the status bubble to be created, removed,
  and have visibility changes.
- The backdrop window itself causes calls to UpdateBackdrop as it
  gets shown and hidden.
- The backdrops weren't showing in the desks mini_views (a
  regression caused by me).
- WorkspaceLayoutManager and Desk were both updating the backdrops
  for the same event.

This CL fixes the above issues.

BUG=1031789
TEST=Manually, all backdrop tests should pass.

Change-Id: I5e25705ae50abdf1cf69b63b01f762044587f600
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974593
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730046}