Oilpan: Prepare moving InspectorController and InspectorAgents to oilpan.

BUG=340522

Review URL: https://codereview.chromium.org/307943002

git-svn-id: svn://svn.chromium.org/blink/trunk@178895 bbb929c8-8fbe-4397-9dbb-9b2b20218538