Add UMA stats for pen palette

ash/common/system/chromeos/palette/common_palette_tool.h

 // 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.
 
 #ifndef ASH_COMMON_SYSTEM_CHROMEOS_PALETTE_COMMON_PALETTE_TOOL_H_
 #define ASH_COMMON_SYSTEM_CHROMEOS_PALETTE_COMMON_PALETTE_TOOL_H_
 
 #include "ash/common/system/chromeos/palette/palette_tool.h"
 #include "ash/common/system/tray/hover_highlight_view.h"
 #include "ash/common/system/tray/view_click_listener.h"
 #include "base/strings/string16.h"
+#include "base/time/time.h"
 #include "ui/gfx/vector_icons_public.h"
 
 namespace ash {
 
 // A PaletteTool implementation with a standard view support.
 class CommonPaletteTool : public PaletteTool, public ash::ViewClickListener {
  protected:
   explicit CommonPaletteTool(Delegate* delegate);
   ~CommonPaletteTool() override;
 
   // PaletteTool:
   views::View* CreateView() override;
   void OnViewDestroyed() override;
   void OnEnable() override;
   void OnDisable() override;
 
   // ViewClickListener:
   void OnViewClicked(views::View* sender) override;
 
   // Returns the icon used in the palette tray on the left-most edge of the
   // tool.
   virtual gfx::VectorIconId GetPaletteIconId() = 0;
 
   // Creates a default view implementation to be returned by CreateView.
   views::View* CreateDefaultView(const base::string16& name);
 
  private:
   HoverHighlightView* highlight_view_ = nullptr;
 
+  // start_time_ is initialized when the tool becomes active.
+  // Used for recording UMA metrics.
+  base::TimeTicks start_time_;
+
   DISALLOW_COPY_AND_ASSIGN(CommonPaletteTool);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_SYSTEM_CHROMEOS_PALETTE_COMMON_PALETTE_TOOL_H_