Bug 1500741 - Support encoding for WebExtension engines. r=mixedpuppy

Differential Revision: https://phabricator.services.mozilla.com/D10941

--HG--
extra : moz-landing-system : lando

browser/components/extensions/schemas/chrome_settings_overrides.json

@@ -101,6 +101,11 @@
                     "optional": true,
                     "deprecated": "Unsupported on Firefox."
                   },
+                  "encoding": {
+                    "type": "string",
+                    "optional": true,
+                    "description": "Encoding of the search term."
+                  },
                   "is_default": {
                     "type": "boolean",
                     "optional": true,

toolkit/components/search/nsSearchService.js

@@ -4004,7 +4004,7 @@ SearchService.prototype = {
       isBuiltIn: extension.isPrivileged,
       suggestURL: searchProvider.suggest_url,
       suggestPostParams: searchProvider.suggest_url_post_params,
-      queryCharset: "UTF-8",
+      queryCharset: searchProvider.encoding || "UTF-8",
       mozParams: searchProvider.params,
     };
     this.addEngineWithDetails(searchProvider.name.trim(), params);