Set HiDpi icons as default for popup

src/manifest.json

@@ -15,7 +15,10 @@
   },
   "default_locale": "en",
   "browser_action": {
-    "default_icon": "images/icon19.png",
+    "default_icon": {
+      "19": "images/icon19.png",
+      "38": "images/icon38.png"
+    },
     "default_title": "__MSG_extName__",
     "default_popup": "popup/index.html"
   },