DevTools: Move _topLevelNodes out of HeapSnapshotViewportDataGrid...

... to a node and rename them into _allChildNodes.

Review URL: https://codereview.chromium.org/183323003

git-svn-id: svn://svn.chromium.org/blink/trunk@168046 bbb929c8-8fbe-4397-9dbb-9b2b20218538