Set keys to traverse accessibility tree. Using focus ring to highlight selected node.

chrome/browser/chromeos/accessibility/select_to_speak_event_handler.cc

Index: chrome/browser/chromeos/accessibility/select_to_speak_event_handler.cc
diff --git a/chrome/browser/chromeos/accessibility/select_to_speak_event_handler.cc b/chrome/browser/chromeos/accessibility/select_to_speak_event_handler.cc
index 62ddc312c419c0806e3160f3ac60ede15079ff95..e4d237ae8fb8e947013ee1efcb0815a1f5857e35 100644
--- a/chrome/browser/chromeos/accessibility/select_to_speak_event_handler.cc
+++ b/chrome/browser/chromeos/accessibility/select_to_speak_event_handler.cc
@@ -5,6 +5,7 @@
 #include "chrome/browser/chromeos/accessibility/select_to_speak_event_handler.h"
 
 #include "ash/shell.h"
+#include "base/logging.h"
 #include "chrome/browser/speech/tts_controller.h"
 #include "chrome/browser/ui/aura/accessibility/automation_manager_aura.h"
 #include "content/public/browser/browser_thread.h"
@@ -28,6 +29,8 @@ SelectToSpeakEventHandler::~SelectToSpeakEventHandler() {
 }
 
 void SelectToSpeakEventHandler::OnKeyEvent(ui::KeyEvent* event) {
+  DCHECK(event);
+
   // We can only call TtsController on the UI thread, make sure we
   // don't ever try to run this code on some other thread.
   CHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
@@ -73,6 +76,7 @@ void SelectToSpeakEventHandler::OnKeyEvent(ui::KeyEvent* event) {
 }
 
 void SelectToSpeakEventHandler::OnMouseEvent(ui::MouseEvent* event) {
+  DCHECK(event);
   if (state_ == INACTIVE)
     return;
 
@@ -157,6 +161,7 @@ void SelectToSpeakEventHandler::OnMouseEvent(ui::MouseEvent* event) {
 }
 
 void SelectToSpeakEventHandler::CancelEvent(ui::Event* event) {
+  DCHECK(event);
   if (event->cancelable()) {
     event->SetHandled();
     event->StopPropagation();