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

ash/common/system/chromeos/palette/tools/capture_region_mode.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/common/system/chromeos/palette/tools/capture_region_mode.h"
 
 #include "ash/common/accelerators/accelerator_controller.h"
 #include "ash/common/palette_delegate.h"
 #include "ash/common/system/chromeos/palette/palette_ids.h"
 #include "ash/common/system/toast/toast_data.h"
@@ skipping 27 matching lines @@
 
 const gfx::VectorIcon& CaptureRegionMode::GetActiveTrayIcon() const {
   return kPaletteTrayIconCaptureRegionIcon;
 }
 
 void CaptureRegionMode::OnEnable() {
   CommonPaletteTool::OnEnable();
 
   ToastData toast(kToastId, l10n_util::GetStringUTF16(
                                 IDS_ASH_STYLUS_TOOLS_CAPTURE_REGION_TOAST),
-                  kToastDurationMs, base::string16());
+                  kToastDurationMs, base::Optional<base::string16>());
   ash::WmShell::Get()->toast_manager()->Show(toast);
 
   WmShell::Get()->palette_delegate()->TakePartialScreenshot(base::Bind(
       &CaptureRegionMode::OnScreenshotDone, weak_factory_.GetWeakPtr()));
   delegate()->HidePalette();
 }
 
 void CaptureRegionMode::OnDisable() {
   CommonPaletteTool::OnDisable();
 
@@ skipping 10 matching lines @@
 const gfx::VectorIcon& CaptureRegionMode::GetPaletteIcon() const {
   return kPaletteActionCaptureRegionIcon;
 }
 
 void CaptureRegionMode::OnScreenshotDone() {
   // The screenshot finished, so disable the tool.
   delegate()->DisableTool(GetToolId());
 }
 
 }  // namespace ash