Change device IDs from unsigned to signed.

ash/display/display_info.h

Index: ash/display/display_info.h
diff --git a/ash/display/display_info.h b/ash/display/display_info.h
index c01d13716dcfba1a889b5b9114b16db2e9ed92f9..427355b801296d1033c0e6da0872d04c3060ed9b 100644
--- a/ash/display/display_info.h
+++ b/ash/display/display_info.h
@@ -111,8 +111,8 @@ class ASH_EXPORT DisplayInfo {
   }
   gfx::Display::TouchSupport touch_support() const { return touch_support_; }
 
-  void set_touch_device_id(unsigned int id) { touch_device_id_ = id; }
-  unsigned int touch_device_id() const { return touch_device_id_; }
+  void set_touch_device_id(int id) { touch_device_id_ = id; }
+  int touch_device_id() const { return touch_device_id_; }
 
   // Gets/Sets the device scale factor of the display.
   float device_scale_factor() const { return device_scale_factor_; }
@@ -236,7 +236,7 @@ class ASH_EXPORT DisplayInfo {
 
   // If the display is also a touch device, it will have a positive
   // |touch_device_id_|. Otherwise |touch_device_id_| is 0.
-  unsigned int touch_device_id_;
+  int touch_device_id_;
 
   // This specifies the device's pixel density. (For example, a
   // display whose DPI is higher than the threshold is considered to have