MD Settings: Factor out some pref manipulation into the PrefsBehavior.

chrome/browser/resources/settings/prefs/prefs_behavior.js

Index: chrome/browser/resources/settings/prefs/prefs_behavior.js
diff --git a/chrome/browser/resources/settings/prefs/prefs_behavior.js b/chrome/browser/resources/settings/prefs/prefs_behavior.js
index 5a4c15d379b9112b08205b1b528a410c0024dc50..8c9446000eff99dfe1d3c4ef2970328a52305880 100644
--- a/chrome/browser/resources/settings/prefs/prefs_behavior.js
+++ b/chrome/browser/resources/settings/prefs/prefs_behavior.js
@@ -32,4 +32,31 @@ var PrefsBehavior = {
     this.getPref(prefPath);  // Ensures we throw if the pref is not found.
     this.set('prefs.' + prefPath + '.value', value);
   },
+
+  /**
+   * Appends the item to the pref list at the given key if the item is not
+   * already in the list. Asserts if the pref itself is not found or is not an
+   * Array type.
+   * @param {string} key
+   * @param {*} item
+   * @protected
+   */
+  appendPrefListItem: function(key, item) {
+    var pref = this.getPref(key);
+    assert(pref && pref.type == chrome.settingsPrivate.PrefType.LIST);
+    if (pref.value.indexOf(item) == -1)
+      this.push('prefs.' + key + '.value', item);
+  },
+
+  /**
+   * Deletes the given item from the pref at the given key if the item is found.
+   * Asserts if the pref itself is not found or is not an Array type.
+   * @param {string} key
+   * @param {*} item
+   * @protected
+   */
+  deletePrefListItem: function(key, item) {
+    assert(this.getPref(key).type == chrome.settingsPrivate.PrefType.LIST);
+    this.arrayDelete('prefs.' + key + '.value', item);
+  },
 };