Fix Scrollbar leak from AccessibilityScrollbar.

The AccessibilityScrollbar leaked reference to Scrollbar as it did not implement detachFromParent().
This bug was apparent when custom scrollbar (RenderScrollbar) was used.

BUG=270000

Review URL: https://chromiumcodereview.appspot.com/23301020

git-svn-id: svn://svn.chromium.org/blink/trunk@156553 bbb929c8-8fbe-4397-9dbb-9b2b20218538