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This CL is the first step that prepares us to get rid of DWTHX11.

As we proceed with that, at some point we will have to throw off the
DesktopDragDropClientAuraX11 because it is used by DWTHX11, and migrate
DnD to DDDCOzone.  As the latter will be used on both Ozone and
non-Ozone, it needs the X11window to handle DnD.

Hence this change.  I move the DnD-related code from X11WindowOzone into
X11Window.

Change-Id: I682c794d9cfab4b7ad6b6a9b89331526f31d227a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2213578Reviewed-by: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Alexander Dunaev <adunaev@igalia.com>
Cr-Commit-Position: refs/heads/master@{#774026}