Add "toOverwrite.filters" entry as managed storage property

The new entry is an array of strings, each representing a
distinct line, and all entries are used to populate the
"My filters" pane.

This offers an more straightforward way for administrators
to specify a list of custom filters to use for all
installations.

platform/chromium/managed_storage.json

@@ -41,6 +41,12 @@
       "title": "Settings to overwrite at launch time",
       "type": "object",
       "properties": {
+        "filters": {
+          "title": "A collection of filters",
+          "description": "The set of user filters to use at launch time -- where each entry is a distinct line.",
+          "type": "array",
+          "items": { "type": "string" }
+        },
         "filterLists": {
           "title": "A collection of list identifiers and/or list URLs",
           "description": "The set of filter lists to use at launch time.",

src/js/storage.js

@@ -1387,7 +1387,12 @@ self.addEventListener('hiddenSettingsChanged', ( ) => {
         vAPI.storage.set(bin);
     }
 
-    if ( typeof data.userFilters === 'string' ) {
+    if (
+        Array.isArray(toOverwrite.filters) &&
+        toOverwrite.filters.length !== 0
+    ) {
+        this.saveUserFilters(toOverwrite.filters.join('\n'));
+    } else if ( typeof data.userFilters === 'string' ) {
         this.saveUserFilters(data.userFilters);
     }
 };