GitLab

This adds initial support for toplevel window dragging during DND
sessions, which is required by Exo's extended-drag protocol impl. To do
so, this CL adds ash::ToplevelWindowDragDelegate, a brand new interface
which makes ash's DragDropController aware of toplevel window dragging
during DND sessions, so allowing it to forward events of interest to its
instance, if it's set for a particular DND operation.

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

The actual ToplevelWindowDragDelegate implementation will be added in a
upcoming CL [2].

Bug: 1099418
Test: ash_unittests --gtest_filter=DragDropControllerTest.ToplevelWIndowDragDelegate

[1] https://docs.google.com/document/d/1s6OwTi_WC-pS21WLGQYI39yw2m42ZlVolUXBclljXB4/edit?usp=sharing
[2] https://chromium-review.googlesource.com/c/chromium/src/+/2401280

R=oshima@chromium.org

Change-Id: I8d76eb7bfe9086f3a89d0c1b8762ea39dbea69a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2483663
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821612}