DevTools: Make ArrayGroupingTreeElement toggleable on click.

All other TreeElement's in ObjectPropertiesSection have this behavior.

R=vsevik

Review URL: https://codereview.chromium.org/513303002

git-svn-id: svn://svn.chromium.org/blink/trunk@181079 bbb929c8-8fbe-4397-9dbb-9b2b20218538