Add UMA stats for pen palette

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

Index: ash/common/system/chromeos/palette/common_palette_tool.h
diff --git a/ash/common/system/chromeos/palette/common_palette_tool.h b/ash/common/system/chromeos/palette/common_palette_tool.h
index 67ef66fc23c22482f24e64bf1702f2db047dcab5..017a0ad3db178aa9e75f0043195355eeb4415400 100644
--- a/ash/common/system/chromeos/palette/common_palette_tool.h
+++ b/ash/common/system/chromeos/palette/common_palette_tool.h
@@ -9,6 +9,7 @@
 #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 {
@@ -38,6 +39,10 @@ class CommonPaletteTool : public PaletteTool, public ash::ViewClickListener {
  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);
 };