[Chrome OS] Add a chrome os device wallpaper policy.

chrome/browser/chromeos/policy/device_policy_decoder_chromeos.cc

 // Copyright (c) 2012 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/chromeos/policy/device_policy_decoder_chromeos.h"
 
 #include <limits>
 #include <string>
 
 #include "base/callback.h"
@@ skipping 822 matching lines @@
     const em::DeviceQuirksDownloadEnabledProto& container(
         policy.quirks_download_enabled());
     if (container.has_quirks_download_enabled()) {
       policies->Set(key::kDeviceQuirksDownloadEnabled, POLICY_LEVEL_MANDATORY,
                     POLICY_SCOPE_MACHINE, POLICY_SOURCE_CLOUD,
                     base::MakeUnique<base::FundamentalValue>(
                         container.quirks_download_enabled()),
                     nullptr);
     }
   }
+
+  if (policy.has_device_wallpaper_image()) {
+    const em::DeviceWallpaperImageProto& container(
+        policy.device_wallpaper_image());
+    if (container.has_device_wallpaper_image()) {
+      std::unique_ptr<base::DictionaryValue> dict_val =
+          base::DictionaryValue::From(
+              base::JSONReader::Read(container.device_wallpaper_image()));
+      policies->Set(key::kDeviceWallpaperImage, POLICY_LEVEL_MANDATORY,
+                    POLICY_SCOPE_MACHINE, POLICY_SOURCE_CLOUD,
+                    std::move(dict_val), nullptr);
+    }
+  }
 }
 
 }  // namespace
 
 void DecodeDevicePolicy(const em::ChromeDeviceSettingsProto& policy,
                         PolicyMap* policies) {
   // Decode the various groups of policies.
   DecodeLoginPolicies(policy, policies);
   DecodeNetworkPolicies(policy, policies);
   DecodeReportingPolicies(policy, policies);
   DecodeAutoUpdatePolicies(policy, policies);
   DecodeAccessibilityPolicies(policy, policies);
   DecodeGenericPolicies(policy, policies);
 }
 
 }  // namespace policy