keyboard: make KeyboardController remove pre-target handler

KeyboardController adds a pre-target handler, but it never removes it.
I'm going to add some DCHECKs to ensure there are no pre-target handlers when
an object is destroyed, so this needs to be fixed.

BUG=867035
TEST=none

Change-Id: Ic8700f1b2433380afaa0fbdff428baf51bfda49b
Reviewed-on: https://chromium-review.googlesource.com/c/1281163Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599756}

ui/keyboard/keyboard_controller.cc

@@ -270,9 +270,13 @@ void KeyboardController::DeactivateKeyboard() {
   // Ensure the keyboard is not visible before deactivating it.
   HideKeyboardExplicitlyBySystem();
 
-  if (GetKeyboardWindow() && GetKeyboardWindow()->parent()) {
-    DCHECK_EQ(parent_container_, GetKeyboardWindow()->parent());
-    parent_container_->RemoveChild(GetKeyboardWindow());
+  aura::Window* keyboard_window = GetKeyboardWindow();
+  if (keyboard_window) {
+    keyboard_window->RemovePreTargetHandler(&event_filter_);
+    if (keyboard_window->parent()) {
+      DCHECK_EQ(parent_container_, keyboard_window->parent());
+      parent_container_->RemoveChild(keyboard_window);
+    }
   }
   parent_container_->GetRootWindow()->RemoveObserver(this);
   parent_container_ = nullptr;