Add metrics for the number of attached displays.

ui/gfx/screen_mac.mm

Index: ui/gfx/screen_mac.mm
diff --git a/ui/gfx/screen_mac.mm b/ui/gfx/screen_mac.mm
index 0b1f41f61d2aee0f9f33c2da94f09b9e5bc07543..1c7638e8b918b6bb59f0c09e3505425da020635a 100644
--- a/ui/gfx/screen_mac.mm
+++ b/ui/gfx/screen_mac.mm
@@ -91,6 +91,11 @@ class ScreenMac : public gfx::Screen {
         ScreenMac::DisplayReconfigurationCallBack, this);
   }
 
+  virtual ~ScreenMac() {
+    CGDisplayRemoveReconfigurationCallback(
+        ScreenMac::DisplayReconfigurationCallBack, this);
+  }
+
   virtual bool IsDIPEnabled() OVERRIDE {
     return true;
   }