Hide dismiss button on toast when doing partial screenshot using stylus tools.

ash/common/system/toast/toast_overlay.h

Index: ash/common/system/toast/toast_overlay.h
diff --git a/ash/common/system/toast/toast_overlay.h b/ash/common/system/toast/toast_overlay.h
index ea2bb391a75349878f3f595f142aee40d5c07f51..4d3fe97fc8a720d81dcf6cd68286c31d9e56864f 100644
--- a/ash/common/system/toast/toast_overlay.h
+++ b/ash/common/system/toast/toast_overlay.h
@@ -8,6 +8,7 @@
 #include <memory>
 
 #include "ash/ash_export.h"
+#include "base/optional.h"
 #include "base/strings/string16.h"
 #include "ui/compositor/layer_animation_observer.h"
 #include "ui/events/event.h"
@@ -38,10 +39,12 @@ class ASH_EXPORT ToastOverlay : public ui::ImplicitAnimationObserver {
 
   // Creates the Toast overlay UI. |text| is the message to be shown, and
   // |dismiss_text| is the message for the button to dismiss the toast message.
-  // |dismiss_text| is optional. If empty, the default text is used.
+  // If |dismiss_text| is null, no dismiss button will be shown. If
+  // |dismiss_text| has a value but the string is empty, the default text is
+  // used.
   ToastOverlay(Delegate* delegate,
                const base::string16& text,
-               const base::string16& dismiss_text);
+               base::Optional<base::string16> dismiss_text);
   ~ToastOverlay() override;
 
   // Shows or hides the overlay.
@@ -58,11 +61,12 @@ class ASH_EXPORT ToastOverlay : public ui::ImplicitAnimationObserver {
   void OnImplicitAnimationsCompleted() override;
 
   views::Widget* widget_for_testing();
+  ToastOverlayButton* dismiss_button_for_testing();
   void ClickDismissButtonForTesting(const ui::Event& event);
 
   Delegate* const delegate_;
   const base::string16 text_;
-  const base::string16 dismiss_text_;
+  const base::Optional<base::string16> dismiss_text_;
   std::unique_ptr<views::Widget> overlay_widget_;
   std::unique_ptr<ToastOverlayView> overlay_view_;
   gfx::Size widget_size_;