Implements a display scale tray setting behind a chrome switch

ui/display/display.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/display/display.h"
 
 #include <algorithm>
 
 #include "base/command_line.h"
 #include "base/logging.h"
@@ skipping 72 matching lines @@
     g_has_forced_device_scale_factor = HasForceDeviceScaleFactorImpl();
   return !!g_has_forced_device_scale_factor;
 }
 
 // static
 void Display::ResetForceDeviceScaleFactorForTesting() {
   g_has_forced_device_scale_factor = -1;
   g_forced_device_scale_factor = -1.0;
 }
 
+// static
+void Display::SetForceDeviceScaleFactor(double dsf) {
+  // Reset any previously set values and unset the flag.
+  g_forced_device_scale_factor = -1.0;
+
+  base::CommandLine::ForCurrentProcess()->AppendSwitchASCII(
+      switches::kForceDeviceScaleFactor, base::StringPrintf("%.2f", dsf));
+}
+
 Display::Display() : Display(kInvalidDisplayId) {}
 
 Display::Display(int64_t id) : Display(id, gfx::Rect()) {}
 
 Display::Display(int64_t id, const gfx::Rect& bounds)
     : id_(id),
       bounds_(bounds),
       work_area_(bounds),
       device_scale_factor_(GetForcedDeviceScaleFactor()),
       color_depth_(DEFAULT_BITS_PER_PIXEL),
@@ skipping 122 matching lines @@
   DCHECK_NE(kInvalidDisplayId, display_id);
   return HasInternalDisplay() && internal_display_id_ == display_id;
 }
 
 // static
 bool Display::HasInternalDisplay() {
   return internal_display_id_ != kInvalidDisplayId;
 }
 
 }  // namespace display