Get the default device scale factor from monitor

ui/aura/aura_switches.cc

Index: ui/aura/aura_switches.cc
diff --git a/ui/aura/aura_switches.cc b/ui/aura/aura_switches.cc
index f0ec044e38a2aaef68f4025afb52ef04e8d7c378..7892e0f0a131dcc6ecf0e819ddcfcda6e64c6cad 100644
--- a/ui/aura/aura_switches.cc
+++ b/ui/aura/aura_switches.cc
@@ -17,4 +17,8 @@ const char kAuraHostWindowSize[] = "aura-host-window-size";
 // Whether to use the full screen for aura's host window.
 const char kAuraHostWindowUseFullscreen[] = "aura-host-window-use-fullscreen";
 
+// The default device scale factor to apply to contents in the absence of
+// a viewport meta tag.
+const char kDefaultDeviceScaleFactor[] = "default-device-scale-factor";
+
 }  // namespace switches