HashTableConstIterator should use Traits::IteratorConstGetType

HashTableConstIterator should use 'Traits::IteratorConstGetType'
and 'Traits::IteratorConstReferenceType' types.
The rationals:
1) Consistency with HashTableIterator
2) That should help to verify that the mentioned types are defined
   properly in the traits classes.
3) HashTableIterator::get() returns 'const_cast' from
   HashTableConstIterator::get(), that might cause compilation
   errors in case 'Traits::IteratorGetType' is not the same
   as 'ValueType*'.

Review URL: https://codereview.chromium.org/171973003

git-svn-id: svn://svn.chromium.org/blink/trunk@167679 bbb929c8-8fbe-4397-9dbb-9b2b20218538