DevTools: fix unjustified modification of sidebar tree when layer tree is not updated

We used to thrash layer tree in the sidebar when receiving unchanged
layer tree with layerTreeDidChange event due to a bug in how we checked
if root has changed parent.

BUG=
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/169753002

git-svn-id: svn://svn.chromium.org/blink/trunk@169088 bbb929c8-8fbe-4397-9dbb-9b2b20218538