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Background:
When dragging home launcher down to show a window, the window will be
shown on drag start and will slide down on drag update. However, this
does not work for window with modal dialog because the dialog's
visibility change cancels the touches for all other windows.

Changes:
Avoid cancel touches on windows that are not in the same transient
window tree of the modal dialog.

Bug: 900321
Change-Id: I2047224de2f6fac9e892376477b7bf9c938cc2c0
Reviewed-on: https://chromium-review.googlesource.com/c/1316434
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607883}