Get the default device scale factor from monitor

ui/base/ui_base_switches.cc

Index: ui/base/ui_base_switches.cc
diff --git a/ui/base/ui_base_switches.cc b/ui/base/ui_base_switches.cc
index d83c3cd214ad9be8e64d2961a3034bf57546c532..d1a2597f5a17c9a41e63ebc635de4aab323a100b 100644
--- a/ui/base/ui_base_switches.cc
+++ b/ui/base/ui_base_switches.cc
@@ -6,6 +6,10 @@
 
 namespace switches {
 
+// The default device scale factor to apply to the browser UI and
+// the contents in the absence of a viewport meta tag.
+const char kDefaultDeviceScaleFactor[] = "default-device-scale-factor";
+
 // Enable support for touch events.
 const char kEnableTouchEvents[]             = "enable-touch-events";