Add defensive null guard to speculatively fix nullptr crash.

Window::GetRootWindow() returns a nullable but there's currently no null
guard nor DCHECK at callsite KeyboardUIController::DeactivateKeyboard().
This is causing some nullptr crash. The crash cannot be repro'd manually
and it's unclear as to why this didn't cause issues earlier.

Bug: 1162338
Change-Id: I40baabcb32aa4d41943af27372c8e2410c2d49f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612587Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Bao-Duy Tran <tranbaoduy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841323}

ash/keyboard/ui/keyboard_ui_controller.cc

@@ -295,7 +295,10 @@ void KeyboardUIController::DeactivateKeyboard() {
       parent_container_->RemoveChild(keyboard_window);
     }
   }
-  parent_container_->GetRootWindow()->RemoveObserver(this);
+  aura::Window* root_window = parent_container_->GetRootWindow();
+  if (root_window) {
+    root_window->RemoveObserver(this);
+  }
   parent_container_ = nullptr;
 }