Reintroduce const qualifiers for return types of (Filter|Transform)Iterator::operator*()
https://bugs.webkit.org/show_bug.cgi?id=126875

The const qualifiers for the return types of FilterIterator::operator*() and TransformIterator::operator*()
were removed in r161797 and r161802 because of compilation failures when using GCC and having an Iterator
type that already had the const qualifier. std::remove_const is now used to appease GCC and enforce the const
qualifier on the return type, regardless of the Iterator type and its qualifiers.

* wtf/IteratorAdaptors.h:
(WTF::FilterIterator::operator*):
(WTF::TransformIterator::operator*):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@164190 268f45cc-cd09-0410-ab3c-d52691b4dbfc