DevTools: avoid using stale front-end node ids.

When a node is removed and then re-added to the document,
the back-end node id remains the same and the front-end id
is new. This means that if we keep resolved front-end nodes,
we should check if these still exist and re-resolve the ids
to new nodes if these are detached and re-attached to the document.

BUG=
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/195813010

git-svn-id: svn://svn.chromium.org/blink/trunk@169526 bbb929c8-8fbe-4397-9dbb-9b2b20218538