cros: Add OEM wallpaper back to wallpaper picker

The added code is mostly the ones deleted in https://chromium-review.googlesource.com/c/chromium/src/+/1297308
as part of "old-wallpaper-picker only" code path.

Bug: 899872
Change-Id: I7dbad13c00d50841091b9f51656428025efae096
Reviewed-on: https://chromium-review.googlesource.com/c/1308936Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604086}

chrome/app/chromeos_strings.grdp

@@ -80,6 +80,9 @@
   <message name="IDS_CURRENT_WALLPAPER_SET_BY_MESSAGE" desc="The string displayed to user if the current wallpaper is set by a third party wallpaper app/extension.">
     The current wallpaper is set by '<ph name="APP_NAME">$1</ph>'. You may need to uninstall '<ph name="APP_NAME">$1</ph>' before selecting a different wallpaper.
   </message>
+  <message name="IDS_DEFAULT_WALLPAPER_ACCESSIBLE_LABEL" desc="Accessibility text read by chromevox when the default wallpaper is focused.">
+    Default wallpaper
+  </message>
 
   <!-- Zip Archiver -->
   <message desc="The product name for Zip Archiver." name="IDS_ZIP_ARCHIVER_NAME">

chrome/browser/chromeos/extensions/wallpaper_private_api.cc

@@ -229,6 +229,7 @@ ExtensionFunction::ResponseAction WallpaperPrivateGetStringsFunction::Run() {
              IDS_WALLPAPER_MANAGER_CONFIRM_PREVIEW_WALLPAPER_LABEL);
   SET_STRING("setSuccessfullyMessage",
              IDS_WALLPAPER_MANAGER_SET_SUCCESSFULLY_MESSAGE);
+  SET_STRING("defaultWallpaperLabel", IDS_DEFAULT_WALLPAPER_ACCESSIBLE_LABEL);
 #undef SET_STRING
 
   const std::string& app_locale = g_browser_process->GetApplicationLocale();

chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_manager.js

@@ -1534,10 +1534,19 @@ WallpaperManager.prototype.onCategoriesChange_ = function() {
     this.wallpaperRequest_ = null;
   }
   if (selectedListItem.custom) {
-    chrome.wallpaperPrivate.getLocalImagePaths(localImagePaths => {
-      // Show a 'no images' message to user if there's no local image.
-      this.updateNoImagesVisibility_(localImagePaths.length == 0);
     var wallpapersDataModel = new cr.ui.ArrayDataModel([]);
+    if (loadTimeData.getBoolean('isOEMDefaultWallpaper')) {
+      var defaultWallpaperInfo = {
+        wallpaperId: null,
+        baseURL: 'OemDefaultWallpaper',
+        layout: Constants.WallpaperThumbnailDefaultLayout,
+        source: Constants.WallpaperSourceEnum.OEM,
+        ariaLabel: loadTimeData.getString('defaultWallpaperLabel'),
+        availableOffline: true
+      };
+      wallpapersDataModel.push(defaultWallpaperInfo);
+    }
+    chrome.wallpaperPrivate.getLocalImagePaths(localImagePaths => {
       for (var imagePath of localImagePaths) {
         var wallpaperInfo = {
           // The absolute file path, used for retrieving the image data if user
@@ -1559,7 +1568,8 @@ WallpaperManager.prototype.onCategoriesChange_ = function() {
         };
         wallpapersDataModel.push(wallpaperInfo);
       }
-      // Display the images.
+      // Show a "no images" message if there's no image.
+      this.updateNoImagesVisibility_(wallpapersDataModel.length == 0);
       this.wallpaperGrid_.dataModel = wallpapersDataModel;
     });
   } else {