This API wasn't working on non chromeos platforms because

it wasn't observing the display change.

BUG=316217
TEST=install the test app attached in the bug and change the scree resolution.

Review URL: https://codereview.chromium.org/581023003

Cr-Commit-Position: refs/heads/master@{#295733}

chrome/browser/extensions/display_info_provider_aura.cc

@@ -4,6 +4,8 @@
 
 #include "chrome/browser/extensions/display_info_provider_aura.h"
 
+#include "ui/gfx/screen.h"
+
 namespace extensions {
 
 DisplayInfoProviderAura::DisplayInfoProviderAura() {
@@ -27,7 +29,7 @@ void DisplayInfoProviderAura::UpdateDisplayUnitInfoForPlatform(
 }
 
 gfx::Screen* DisplayInfoProviderAura::GetActiveScreen() {
-  return NULL;
+  return gfx::Screen::GetNativeScreen();
 }
 
 // static

chrome/browser/extensions/display_info_provider_mac.cc

@@ -4,6 +4,8 @@
 
 #include "chrome/browser/extensions/display_info_provider_mac.h"
 
+#include "ui/gfx/screen.h"
+
 namespace extensions {
 
 DisplayInfoProviderMac::DisplayInfoProviderMac() {
@@ -27,7 +29,7 @@ void DisplayInfoProviderMac::UpdateDisplayUnitInfoForPlatform(
 }
 
 gfx::Screen* DisplayInfoProviderMac::GetActiveScreen() {
-  return NULL;
+  return gfx::Screen::GetNativeScreen();
 }
 
 // static

chrome/browser/extensions/display_info_provider_win.cc

@@ -11,6 +11,7 @@
 #include "base/strings/utf_string_conversions.h"
 #include "base/win/win_util.h"
 #include "extensions/common/api/system_display.h"
+#include "ui/gfx/screen.h"
 #include "ui/gfx/size.h"
 #include "ui/gfx/win/dpi.h"
 
@@ -81,7 +82,8 @@ void DisplayInfoProviderWin::UpdateDisplayUnitInfoForPlatform(
 }
 
 gfx::Screen* DisplayInfoProviderWin::GetActiveScreen() {
-  return NULL;
+  // TODO(scottmg): native screen is wrong http://crbug.com/133312
+  return gfx::Screen::GetNativeScreen();
 }
 
 // static