vk: Speculative fix for keyboard crash.

Chrome seems to crash when adding a new monitor with the virtual
keyboard enabled. The stack trace suggests the keyboard window is null.
Check for when the keyboard window is null as a speculative fix.

Bug: 1157150
Change-Id: I6475dde46bed1a9c8f5246ace76790e836a312e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2585262Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836842}

ash/keyboard/ui/keyboard_ui_controller.cc

@@ -965,7 +965,9 @@ void KeyboardUIController::ReportLingeringState() {
 }
 
 gfx::Rect KeyboardUIController::GetWorkspaceOccludedBoundsInScreen() const {
-  if (!ui_)
+  // TODO(crbug.com/1157150): Investigate why the keyboard window may become
+  // null when adding a new monitor.
+  if (!ui_ || !GetKeyboardWindow())
     return gfx::Rect();
 
   const gfx::Rect visual_bounds_in_window(visual_bounds_in_root_.size());