Hookup device dpi for ChromeOS

chrome/browser/extensions/display_info_provider_chromeos.cc

 // Copyright 2014 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 "chrome/browser/extensions/display_info_provider_chromeos.h"
 
 #include "ash/display/display_manager.h"
 #include "ash/display/window_tree_host_manager.h"
 #include "ash/shell.h"
 #include "base/strings/string_number_conversions.h"
@@ skipping 348 matching lines @@
     const gfx::Display& display,
     extensions::api::system_display::DisplayUnitInfo* unit) {
   ash::DisplayManager* display_manager =
       ash::Shell::GetInstance()->display_manager();
   unit->name = display_manager->GetDisplayNameForId(display.id());
   if (display_manager->IsInMirrorMode()) {
     unit->mirroring_source_id =
         base::Int64ToString(display_manager->mirroring_display_id());
   }
 
-  // TODO(hshi): determine the DPI of the screen.
-  const float kDpi96 = 96.0;
-
-  const float dpi = display.device_scale_factor() * kDpi96;
+  const ash::DisplayInfo& display_info = display_manager->GetDisplayInfo(
+      display.id());
+  const float dpi = display_info.device_dpi() / display.device_scale_factor();
   unit->dpi_x = dpi;
   unit->dpi_y = dpi;
 
   const gfx::Insets overscan_insets =
       display_manager->GetOverscanInsets(display.id());
   unit->overscan.left = overscan_insets.left();
   unit->overscan.top = overscan_insets.top();
   unit->overscan.right = overscan_insets.right();
   unit->overscan.bottom = overscan_insets.bottom();
 }
 
 gfx::Screen* DisplayInfoProviderChromeOS::GetActiveScreen() {
   return ash::Shell::GetScreen();
 }
 
 // static
 DisplayInfoProvider* DisplayInfoProvider::Create() {
   return new DisplayInfoProviderChromeOS();
 }
 
 }  // namespace extensions