[DevTools] Resources panel SideBarTreeElement should show expand/collapse option only if it has children

Currently Resources panel sidebar tree elements show expand/collapse option irrespective of
them having children or not. This could be misunderstood as an error in inspector.

This change fixes the same by providing expand/collapse option only when the tree elements
have children.

R=pfeldman

Review URL: https://chromiumcodereview.appspot.com/24109003

git-svn-id: svn://svn.chromium.org/blink/trunk@157951 bbb929c8-8fbe-4397-9dbb-9b2b20218538