TimelinePanel: rename elementId and rootNode into backendNodeId and remove it from the top level properties of TimelineModel.Record

BUG=361045
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/245153003

git-svn-id: svn://svn.chromium.org/blink/trunk@172354 bbb929c8-8fbe-4397-9dbb-9b2b20218538