Bug 826539 - Part 0: use BrowserContract constants instead of literal GUIDs.
Bug 826539 - Part 1: forbid syncing the Pinned Items folder.
Bug 826539 - Part 2: generalize folder child skipping from reading list to include pinned items.
Bug 826539 - Part 3: test that pinned items are ignored.
Bug 826539 - Pinned tabs folder shouldn't be synced. r=nalexander