Enable touch for pagination indicators on launcher

Changes:
Enable touch event in PageSwitcherButton.

BUG=750658

Change-Id: I293544abf5d90201687800d6d6d0451810dfaacd
Reviewed-on: https://chromium-review.googlesource.com/595127Reviewed-by: Yury Khmel <khmel@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491629}

ui/app_list/views/page_switcher_vertical.cc

@@ -72,11 +72,6 @@ class PageSwitcherButton : public views::CustomButton {
   }
 
  protected:
-  bool IsTriggerableEvent(const ui::Event& event) override {
-    return event.IsMouseEvent() &&
-           (triggerable_event_flags() & event.flags()) != 0;
-  }
-
   std::unique_ptr<views::InkDrop> CreateInkDrop() override {
     std::unique_ptr<views::InkDropImpl> ink_drop =
         CustomButton::CreateDefaultInkDropImpl();