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 2cefaffaf64b35e652d485b8c454b0cd3fa4c3a7..fb9419cb5558910462e2a37e050ad5b606098531 100644
--- a/ash/common/system/chromeos/palette/common_palette_tool.h
+++ b/ash/common/system/chromeos/palette/common_palette_tool.h
@@ -8,6 +8,7 @@
 #include "ash/common/system/chromeos/palette/palette_tool.h"
 #include "ash/common/system/tray/view_click_listener.h"
 #include "base/strings/string16.h"
+#include "base/time/time.h"
 
 namespace gfx {
 struct VectorIcon;
@@ -42,6 +43,10 @@ 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);
 };