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* I uesd ShelfBubbleContainer because it has to be above most of
  windows, but below lock screen.
* Refactor ShellSurfaceBase::CreateShellSurfaceWidget
  - use parent_ only if the container isn't specified.
* Removed SetContainer(menu_container) in zxdg_shell.cc because
  it's unnecessary. (It was using transient parent, and container_ was ignored)

We may need a separate container for override redirect but
we probably should implement the xgrabpointer correctly first.

Bug: 938130

Change-Id: I2e24e3f117764ff2e420fb9f7bd6390673d25c83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1602659Reviewed-by: Nic Hollingum <hollingum@google.com>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659385}