Ease expectation of having move/resize handler.

It is not guaranteed that move/resize handler is set by a
PlatformWindow. Thus, remove the DCHECK and add if (handler_)
condition as part of the existing condition to dispatch move/resize
requests.

Bug: 990756
Change-Id: Ibfa1251f65af547382f04d44fdef28d620c0bdd1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1807311Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#697919}

ui/views/widget/desktop_aura/window_event_filter_linux.cc

@@ -25,9 +25,7 @@ namespace views {
 WindowEventFilterLinux::WindowEventFilterLinux(
     DesktopWindowTreeHost* window_tree_host,
     ui::WmMoveResizeHandler* handler)
-    : window_tree_host_(window_tree_host), handler_(handler) {
-  DCHECK(handler_);
-}
+    : window_tree_host_(window_tree_host), handler_(handler) {}
 
 WindowEventFilterLinux::~WindowEventFilterLinux() = default;
 
@@ -154,7 +152,8 @@ void WindowEventFilterLinux::LowerWindow() {}
 void WindowEventFilterLinux::MaybeDispatchHostWindowDragMovement(
     int hittest,
     ui::MouseEvent* event) {
-  if (event->IsLeftMouseButton() && ui::CanPerformDragOrResize(hittest)) {
+  if (handler_ && event->IsLeftMouseButton() &&
+      ui::CanPerformDragOrResize(hittest)) {
     // Some platforms (eg X11) may require last pointer location not in the
     // local surface coordinates, but rather in the screen coordinates for
     // interactive move/resize.