Enable user change background image in settings page in Aura build.

ash/accelerators/accelerator_controller.cc

Index: ash/accelerators/accelerator_controller.cc
diff --git a/ash/accelerators/accelerator_controller.cc b/ash/accelerators/accelerator_controller.cc
index 1df6887fd1a8cd0619651af764c031bd1acdfb36..08aa2308f277e902de17b81b7c43834c6bb93ec3 100644
--- a/ash/accelerators/accelerator_controller.cc
+++ b/ash/accelerators/accelerator_controller.cc
@@ -199,9 +199,12 @@ bool HandleRotateScreen() {
 bool HandleToggleDesktopBackgroundMode() {
   ash::Shell* shell = ash::Shell::GetInstance();
   if (shell->desktop_background_mode() == ash::Shell::BACKGROUND_IMAGE)
-    shell->SetDesktopBackgroundMode(ash::Shell::BACKGROUND_SOLID_COLOR);
-  else
-    shell->SetDesktopBackgroundMode(ash::Shell::BACKGROUND_IMAGE);
+    shell->SetDesktopBackgroundSolidColor();
+  else {
+    shell->SetDesktopBackgroundImage(ash::Shell::GetInstance()->
+                                     desktop_background_controller()->
+                                     GetUserWallpaper());
+  }
   return true;
 }