ozone/wayland: Destroy offer if its target surface gets destroyed before enter event

During Chrome's tab dragging, when a browser window is quickly snapped
in and out, its surface and xdg surface might get destroyed before the
wl_data_device::enter event is processed for the drag offer for that
give surface.

If that happens, the offer object must be destroyed right away, as there
will be no wl_data_device::leave event for that surface (where it would
destroyed otherwise) and some compositors usually assume there is no
active data offer when a new surface is entered. Such behavior has been
observed in Exosphere compositor, for example [1].

[1] https://source.chromium.org/chromium/chromium/src/+/master:components/exo/data_device.cc;l=77;drc=abf7a4e9b3d2151e0c5c05cf8603e9029f4e006d

R=msisov@igalia.com

Bug: 896640
Change-Id: I88668d44ccdc1507fdebc4fdbe2b676d8848c978
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2378620
Commit-Queue: Maksim Sisov (GMT+3) <msisov@igalia.com>
Reviewed-by: Maksim Sisov (GMT+3) <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#802139}

ui/ozone/platform/wayland/host/wayland_data_device.cc

@@ -109,14 +109,20 @@ void WaylandDataDevice::OnEnter(void* data,
                                 wl_fixed_t x,
                                 wl_fixed_t y,
                                 wl_data_offer* offer) {
+  auto* self = static_cast<WaylandDataDevice*>(data);
   WaylandWindow* window = wl::RootWindowFromWlSurface(surface);
+
+  // During Chrome's tab dragging, when a browser window is quickly snapped in
+  // and out, it might get destroyed before the wl_data_device::enter event is
+  // processed for a drag offer. If this happens, |window| will be null here, so
+  // destroy |new_offer_| here, as some compositors assume it. Such behavior has
+  // been observed in Exosphere compositor, for example.
   if (!window) {
-    LOG(ERROR) << "Failed to get window.";
+    self->new_offer_.reset();
+    VLOG(1) << "Failed to get window.";
     return;
   }
 
-  auto* self = static_cast<WaylandDataDevice*>(data);
-
   // Null |drag_delegate_| here means that the DND session has been initiated by
   // an external application. In this case, use the default data drag delegate.
   if (!self->drag_delegate_)