Implements a display scale tray setting behind a chrome switch

ui/display/display.cc

Index: ui/display/display.cc
diff --git a/ui/display/display.cc b/ui/display/display.cc
index 25573f8f9b90e3408ab9234162a0a9b0c92aae70..4903a223a2e83504178e42aec18e4d985796ae16 100644
--- a/ui/display/display.cc
+++ b/ui/display/display.cc
@@ -90,6 +90,22 @@ void Display::ResetForceDeviceScaleFactorForTesting() {
   g_forced_device_scale_factor = -1.0;
 }
 
+// static
+void Display::SetForceDeviceScaleFactor(double dsf) {
+  std::string str_value = base::DoubleToString(dsf);
+
+  // We need a precision of upto 2 decimal point.
+  if (str_value.length() > 4)
+    str_value.erase(str_value.begin() + 4, str_value.end());
+
+  // Reset any previously set values and unset the flag.
+  g_has_forced_device_scale_factor = -1;
+  g_forced_device_scale_factor = -1.0;
+
+  base::CommandLine::ForCurrentProcess()->AppendSwitchASCII(
+      switches::kForceDeviceScaleFactor, str_value);
+}
+
 Display::Display() : Display(kInvalidDisplayId) {}
 
 Display::Display(int64_t id) : Display(id, gfx::Rect()) {}