Remove SetFocusBehavior from MessageCenterScrollView.

MessageCenterScrollView need not be focused.

This CL also fixes a bug, tab focus missing on the UI.

BUG=807534
TEST=manual

Change-Id: I5427e84c49c2bbfbfa90944c334ffa5aa805d8d9
Reviewed-on: https://chromium-review.googlesource.com/946068Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Megumi Hattori <megumihattori@google.com>
Cr-Commit-Position: refs/heads/master@{#541706}

ash/message_center/message_center_view.cc

@@ -590,16 +590,6 @@ void MessageCenterView::Update(bool animate) {
   else
     SetVisibilityMode(Mode::NOTIFICATIONS, animate);
 
-  if (no_message_views) {
-    scroller_->SetFocusBehavior(FocusBehavior::NEVER);
-  } else {
-#if defined(OS_MACOSX)
-    scroller_->SetFocusBehavior(FocusBehavior::ACCESSIBLE_ONLY);
-#else
-    scroller_->SetFocusBehavior(FocusBehavior::ALWAYS);
-#endif
-  }
-
   UpdateButtonBarStatus();
 
   if (scroller_->visible())