Oilpan: Remove RefPtrs to Node and its subclasses in core/inspector/ with Oilpan transition types.

InspectorDOMAgent::m_documentNodeToIdMap needs to be switched from a part object
to OwnPtr/Persistent because we have some addres comparisons of
NodeToIdMap(HeapHashMap) and we have no ways to get a HeapHashMap pointer from a
PersistentHeapHashMap.

BUG=357163

Review URL: https://codereview.chromium.org/315213006

git-svn-id: svn://svn.chromium.org/blink/trunk@175662 bbb929c8-8fbe-4397-9dbb-9b2b20218538