DevTools: transition from DOMAgent's backend node ids to InspectorNodeIds

Use InspectorNodeIds where DOMAgent::backendNodeIdForNode() was previously
used. Drop DOMAgent's support for backend node ids, except for
pushNodeByBackendIdsToFrontend(). The latter now returns 0 for ids that it
fails to resolve if these were released or do not belong to the inspected page.

BUG=361045

Review URL: https://codereview.chromium.org/265293002

git-svn-id: svn://svn.chromium.org/blink/trunk@173318 bbb929c8-8fbe-4397-9dbb-9b2b20218538