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 c07dd8023066620c84960ea39afec287dcc062ca..cebefa2e255dd0d2d24031d80640ab411ac27319 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;
   }