Use 'HashTraits::PeekInType' in the ListHashSet 'add', 'append' and 'insert' methods

Use the 'HashTraits::PeekInType' as an argument type
in ListHashSet 'add', 'append' and 'insert' methods
(same way it is done now in HashMap and HashSet) thus
enabling possible optimizations (e.g. 'RefPtrValuePeeker'
for 'RefPtr' argument type).

Review URL: https://codereview.chromium.org/199303002

git-svn-id: svn://svn.chromium.org/blink/trunk@169195 bbb929c8-8fbe-4397-9dbb-9b2b20218538