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When initializing a new widget such that its native window is
added as a transient child of a window tracked by a WindowPreviewView,
WindowPreviewView gets notified with this addition *before* the window
is parented to a container. If we try to get its WindowState at this
point we would crash at this line: https://cs.chromium.org/chromium/src/ash/wm/window_state.cc?rcl=110515d4d38fd48cb9c6a1d0d3e4f9abf7c415f4&l=820

This CL fixes this issue and adds a test that fails without the fix.

BUG=1003544
TEST=Added a new test

Change-Id: I794d312601eb5b80f1928e86ba0fa42911b8c327
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1826031
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702112}