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Context: Wayland Protocol needs to be extended to make it possible to
properly support full Chrome's tab dragging experience. Further details
in the Design document [1].

This is the first of a patch series which implements extended-drag in
Exo compositor. More specifically, this implements the core feature of
the protocol: Making it possible to drag toplevel shell surfaces during
DND sessions. It does it by making ExtendedDragSource an implementation
of the ash::ToplevelWindowDragDelegate interface and plumbing it into
ash's ToplevelWindowEventHandler, delegating to it the actual toplevel
window drag handling. Refer to the design doc for more details.

[1] https://docs.google.com/document/d/1s6OwTi_WC-pS21WLGQYI39yw2m42ZlVolUXBclljXB4/edit?usp=sharing

R=oshima@chromium.org

Bug: 1099418
Test: exo_unittests --gtest_filter='ExtendedDragSourceTest.*'
Change-Id: I43908dda9f542e5813d96a2e331af8df0a703efb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401280Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#822682}