Oilpan: Avoid declaring a destructor for garbage collected ListHashSet.

R=erik.corry@gmail.com, tkent@chromium.org
BUG=

Review URL: https://codereview.chromium.org/274023002

git-svn-id: svn://svn.chromium.org/blink/trunk@173746 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Oilpan: Prepare to move IdTargetObserver and IdTargetObserverRegistry to Oilpan heap.

BUG=357163

Review URL: https://codereview.chromium.org/278803002

git-svn-id: svn://svn.chromium.org/blink/trunk@173747 bbb929c8-8fbe-4397-9dbb-9b2b20218538