Prepare ScreenWin To Use Per-Monitor DPI

ui/display/win/screen_win.cc

Index: ui/display/win/screen_win.cc
diff --git a/ui/display/win/screen_win.cc b/ui/display/win/screen_win.cc
index 3b6f6dfcb4ee37bb30a94ecbad8741a10a435057..d84926b5ad23897bd219c0c5c1f31eecd733f808 100644
--- a/ui/display/win/screen_win.cc
+++ b/ui/display/win/screen_win.cc
@@ -5,11 +5,13 @@
 #include "ui/display/win/screen_win.h"
 
 #include <windows.h>
+#include <shellscalingapi.h>
 
 #include <algorithm>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
+#include "base/win/win_util.h"
 #include "ui/display/display.h"
 #include "ui/display/manager/display_layout.h"
 #include "ui/display/manager/display_layout_builder.h"
@@ -30,6 +32,31 @@ namespace {
 // resolved with Desktop Aura and WindowTreeHost.
 ScreenWin* g_screen_win_instance = nullptr;
 
+float GetMonitorScaleFactor(HMONITOR monitor) {
+  DCHECK(monitor);
+  if (base::win::IsProcessPerMonitorDpiAware()) {
+    static auto get_dpi_for_monitor_func = [](){
+      using GetDpiForMonitorPtr = decltype(::GetDpiForMonitor)*;
+      HMODULE shcore_dll = ::LoadLibrary(L"shcore.dll");
+      if (shcore_dll) {
+        return reinterpret_cast<GetDpiForMonitorPtr>(
+                   ::GetProcAddress(shcore_dll, "GetDpiForMonitor"));
+      }
+      return reinterpret_cast<GetDpiForMonitorPtr>(nullptr);
+    }();
+
+    UINT dpi_x;
+    UINT dpi_y;
+    if (get_dpi_for_monitor_func &&
+        SUCCEEDED(get_dpi_for_monitor_func(monitor, MDT_EFFECTIVE_DPI,
+                                           &dpi_x, &dpi_y))) {
+      DCHECK_EQ(dpi_x, dpi_y);
+      return GetScalingFactorFromDPI(dpi_x);
+    }
+  }
+  return GetDPIScale();
+}
+
 std::vector<DisplayInfo> FindAndRemoveTouchingDisplayInfos(
     const DisplayInfo& ref_display_info,
     std::vector<DisplayInfo>* display_infos) {
@@ -144,10 +171,8 @@ BOOL CALLBACK EnumMonitorCallback(HMONITOR monitor,
   std::vector<DisplayInfo>* display_infos =
       reinterpret_cast<std::vector<DisplayInfo>*>(data);
   DCHECK(display_infos);
-  // TODO(robliao): When ready, replace the GetDPIScale with GetDpiForMonitor
-  // to get the actual DPI for the HMONITOR.
   display_infos->push_back(DisplayInfo(MonitorInfoFromHMONITOR(monitor),
-                                       GetDPIScale()));
+                                       GetMonitorScaleFactor(monitor)));
   return TRUE;
 }