Validate the Preferences when fetching them from storage

When updating Preferences using the `set` method, the input is carefully validated. However, no validation is (currently) done when the Preferences are initially read, which probably isn't that great; hence this patch that simply ignores invalid Preferences.

web/preferences.js

@@ -57,12 +57,30 @@ class BasePreferences {
         enumerable: true,
         configurable: false,
       });
+      return this._initPrefs();
+    });
+  }
+
+  /**
+   * @private
+   */
+  _initPrefs() {
+    const defaults = this.defaults;
+    this.prefs = Object.assign(Object.create(null), defaults);
 
-      this.prefs = Object.assign(Object.create(null), defaults);
-      return this._readFromStorage(defaults);
-    }).then((prefObj) => {
-      if (prefObj) {
-        this.prefs = prefObj;
+    return this._readFromStorage(defaults).then((prefObj) => {
+      if (!prefObj) {
+        return;
+      }
+      for (let name in prefObj) {
+        const defaultValue = defaults[name], prefValue = prefObj[name];
+        // Ignore preferences not present in, or whose types don't match,
+        // the default values.
+        if (defaultValue === undefined ||
+            typeof prefValue !== typeof defaultValue) {
+          continue;
+        }
+        this.prefs[name] = prefValue;
       }
     });
   }
@@ -106,11 +124,7 @@ class BasePreferences {
    */
   reload() {
     return this._initializedPromise.then(() => {
-      return this._readFromStorage(this.defaults);
-    }).then((prefObj) => {
-      if (prefObj) {
-        this.prefs = prefObj;
-      }
+      return this._initPrefs();
     });
   }