Update Wallpaper Manager icons for MD compliance

Added 48px variant to avoid excessive downscaling.

Bug: 882176
Test: manual
Change-Id: Icfe9e1ea2615987e19b0ac186d2c34b9e8ff5898
Reviewed-on: https://chromium-review.googlesource.com/1219493Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590399}

chrome/browser/resources/chromeos/wallpaper_manager/manifest.json

@@ -8,6 +8,7 @@
   "manifest_version": 2,
   "description": "An experimental wallpaper picker UI",
   "icons": {
+    "48": "images/icon48.png",
     "192": "images/icon192.png"
   },
   "permissions": [