Remove WeakNodeMap from oilpan bulids

- WeakNodeMap.{h,cpp} is only used by InspectorNodeIds.{h,cpp}.

- WeakNodeMap is not needed at all in oilpan bulids, because in oilpan we can just use on-heap hash maps of weak members.

- This CL removes WeakNodeMap from oilpan builds. As far as I can tell, the simplest way of doing that is just to comment out WeakNodeMap using #if !ENABLE(OILPAN) and add another implementation of InspectorNodeIds' methods for ENABLE(OILPAN).

BUG=340522

Review URL: https://codereview.chromium.org/387473002

git-svn-id: svn://svn.chromium.org/blink/trunk@177831 bbb929c8-8fbe-4397-9dbb-9b2b20218538