Introduce user customization of external HighDPI mode for 4K monitor.

ash/display/display_manager.cc

Index: ash/display/display_manager.cc
diff --git a/ash/display/display_manager.cc b/ash/display/display_manager.cc
index 07d44155fa6ccdd1ad4a6e89b7d7939a69455f5e..b7538e7ac896674c4b67ac7df7f5162015b156ca 100644
--- a/ash/display/display_manager.cc
+++ b/ash/display/display_manager.cc
@@ -4,6 +4,7 @@
 
 #include "ash/display/display_manager.h"
 
+#include <algorithm>
 #include <cmath>
 #include <set>
 #include <string>
@@ -472,6 +473,50 @@ void DisplayManager::SetDisplayResolution(int64 display_id,
 #endif
 }
 
+void DisplayManager::SetDisplayMode(int64 display_id,
+                                    const DisplayMode& display_mode) {
+  if (IsInternalDisplayId(display_id)) {
+    SetDisplayUIScale(display_id, display_mode.ui_scale);
+    return;
+  }
+
+  DisplayInfoList display_info_list;
+  bool display_property_changed = false;
+  bool resolution_changed = false;
+  for (DisplayList::const_iterator iter = displays_.begin();
+       iter != displays_.end(); ++iter) {
+    DisplayInfo info = GetDisplayInfo(iter->id());
+    if (info.id() == display_id) {
+      if (info.bounds_in_native().size() != display_mode.size) {
+        const std::vector<DisplayMode>& modes = info.display_modes();
+        std::vector<DisplayMode>::const_iterator iter =
+            std::find_if(modes.begin(), modes.end(),
+                         DisplayModeMatcher(display_mode.size));
+        if (iter == modes.end()) {
+          LOG(WARNING) << "Unsupported resolution was requested:"
+                       << display_mode.size.ToString();
+          return;
+        }
+        display_modes_[display_id] = *iter;
+        resolution_changed = true;
+      }
+      if (info.device_scale_factor() != display_mode.device_scale_factor) {
+        info.set_device_scale_factor(display_mode.device_scale_factor);
+        display_property_changed = true;
+      }
+    }
+    display_info_list.push_back(info);
+  }
+  if (display_property_changed) {
+    AddMirrorDisplayInfoIfAny(&display_info_list);
+    UpdateDisplays(display_info_list);
+  }
+#if defined(OS_CHROMEOS)
+  if (resolution_changed && base::SysInfo::IsRunningOnChromeOS())
+    Shell::GetInstance()->display_configurator()->OnConfigurationChanged();
+#endif
+}
+
 void DisplayManager::RegisterDisplayProperty(
     int64 display_id,
     gfx::Display::Rotation rotation,