[ozone/x11] Fixed calling OnDragFinished with the appropriate result.

The drag handler delegate should receive the resulting drag and drop
operation via its OnDragFinished, but the Ozone/X11 implementation
returned 0 always.  This CL fixes that.

Bug: 1014860
Change-Id: Ie339b7c7beb374bc58d08a332c2b817b8dc0b04f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2253738Reviewed-by: Maksim Sisov <msisov@igalia.com>
Commit-Queue: Alexander Dunaev <adunaev@igalia.com>
Cr-Commit-Position: refs/heads/master@{#780207}

ui/platform_window/x11/x11_window.cc

@@ -883,7 +883,7 @@ int X11Window::PerformDrop() {
 
 void X11Window::EndDragLoop() {
   DCHECK(drag_handler_delegate_);
-  drag_handler_delegate_->OnDragFinished(0);
+  drag_handler_delegate_->OnDragFinished(drag_operation_);
   drag_handler_delegate_ = nullptr;
 }
 #endif  // defined(USE_OZONE)