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..622630caf5e98d3db1293d1c1ecdb866ca2280cc 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>
@@ -68,6 +69,7 @@ const float kUIScalesFor2x[] =
 const float kUIScalesFor1_25x[] = {0.5f, 0.625f, 0.8f, 1.0f, 1.25f };
 const float kUIScalesFor1280[] = {0.5f, 0.625f, 0.8f, 1.0f, 1.125f };
 const float kUIScalesFor1366[] = {0.5f, 0.6f, 0.75f, 1.0f, 1.125f };
+const float kDeviceScaleFactorsFor4k[] = {1.0f, 1.25f, 2.0f};
 
 struct DisplaySortFunctor {
   bool operator()(const gfx::Display& a, const gfx::Display& b) {
@@ -87,6 +89,10 @@ struct DisplayModeMatcher {
   gfx::Size size;
 };
 
+bool CompareDisplayModeBySize(const DisplayMode& m1, const DisplayMode& m2) {
+  return m1.size.GetArea() < m2.size.GetArea();
+}
+
 struct ScaleComparator {
   explicit ScaleComparator(float s) : scale(s) {}
 
@@ -213,6 +219,24 @@ std::vector<float> DisplayManager::GetScalesForDisplay(
 }
 
 // static
+std::vector<float> DisplayManager::GetDeviceScaleFactorsForDisplay(
+    const DisplayInfo& info) {
+  // This method is currently only used for external displays.
+  std::vector<float> scale_factors;
+  scale_factors.push_back(1.0f);
+  if (gfx::Display::InternalDisplayId() == info.id())
+    return scale_factors;
+
+  const std::vector<DisplayMode>& display_modes = info.display_modes();
+  std::vector<DisplayMode>::const_iterator iter =
+      std::max_element(display_modes.begin(), display_modes.end(),
+                       CompareDisplayModeBySize);
+  if (iter != display_modes.end() && iter->size.width() >= 3840)
+    ASSIGN_ARRAY(scale_factors, kDeviceScaleFactorsFor4k);
+  return scale_factors;
+}
+
+// static
 float DisplayManager::GetNextUIScale(const DisplayInfo& info, bool up) {
   float scale = info.configured_ui_scale();
   std::vector<float> scales = GetScalesForDisplay(info);
@@ -472,6 +496,32 @@ void DisplayManager::SetDisplayResolution(int64 display_id,
 #endif
 }
 
+void DisplayManager::SetDisplayDeviceScaleFactor(int64 display_id,
+                                                 float device_scale_factor) {
+  DCHECK_NE(gfx::Display::InternalDisplayId(), display_id);
+  if (gfx::Display::InternalDisplayId() == display_id)
+    return;
+
+  DisplayInfoList display_info_list;
+  for (DisplayList::const_iterator iter = displays_.begin();
+       iter != displays_.end(); ++iter) {
+    DisplayInfo info = GetDisplayInfo(iter->id());
+    if (info.id() == display_id) {
+      if (info.device_scale_factor() == device_scale_factor)
+        return;
+      std::vector<float> scales = GetDeviceScaleFactorsForDisplay(info);
+      ScaleComparator comparator(device_scale_factor);
+      if (std::find_if(scales.begin(), scales.end(), comparator) ==
+          scales.end()) {
+        return;
+      }
+      info.set_device_scale_factor(device_scale_factor);
+    }
+    display_info_list.push_back(info);
+  }
+  UpdateDisplays(display_info_list);
+}
+
 void DisplayManager::RegisterDisplayProperty(
     int64 display_id,
     gfx::Display::Rotation rotation,