Fix crash on disabling Select-to-Speak.

Actually remove the Select-to-Speak event handler instead of
trying to add it a second time.

TBR=dmazzoni@chromium.org

Change-Id: Ie8a8dd15adfa16e58ef8d46c60ba3f9b3471b0e6
Reviewed-on: https://chromium-review.googlesource.com/993473Reviewed-by: Katie Dektar <katie@chromium.org>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547808}

chrome/browser/chromeos/accessibility/select_to_speak_event_handler.cc

@@ -59,8 +59,7 @@ SelectToSpeakEventHandler::SelectToSpeakEventHandler() {
 
 SelectToSpeakEventHandler::~SelectToSpeakEventHandler() {
   if (aura::Env::GetInstanceDontCreate())
-    aura::Env::GetInstanceDontCreate()->AddPreTargetHandler(
-        this, ui::EventTarget::Priority::kAccessibility);
+    aura::Env::GetInstanceDontCreate()->RemovePreTargetHandler(this);
 }
 
 void SelectToSpeakEventHandler::CaptureForwardedEventsForTesting(