[ozone/wayland] Do not add window if manager does not exist

It may happen that the zwp linux dmabuf protocol is not available.
In this case, we have already been resetting the gbm device and
make the gpu use wl_egl instead, but the WaylandConnection still
has been trying to add a window to the WaylandBufferManager upon
creation. However, as previously said, the manager may not exist
because the zwp linux dmabuf had not been advertised as supported
protocol.

Without this patch, it just crashes.

Bug: 578890
Change-Id: I15bcd7ae968590d358a4590d79a54067de02e8d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609822Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#659405}

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

@@ -160,8 +160,10 @@ WaylandWindow* WaylandConnection::GetCurrentKeyboardFocusedWindow() const {
 
 void WaylandConnection::AddWindow(gfx::AcceleratedWidget widget,
                                   WaylandWindow* window) {
-  DCHECK(buffer_manager_);
-  buffer_manager_->OnWindowAdded(window);
+  if (buffer_manager_) {
+    DCHECK(zwp_dmabuf_);
+    buffer_manager_->OnWindowAdded(window);
+  }
 
   window_map_[widget] = window;
 }
@@ -170,8 +172,8 @@ void WaylandConnection::RemoveWindow(gfx::AcceleratedWidget widget) {
   if (touch_)
     touch_->RemoveTouchPoints(window_map_[widget]);
 
-  DCHECK(buffer_manager_);
-  buffer_manager_->OnWindowRemoved(window_map_[widget]);
+  if (buffer_manager_)
+    buffer_manager_->OnWindowRemoved(window_map_[widget]);
 
   window_map_.erase(widget);
 }