Fix ignored const qualifier in HashTraits.h

A new version of Clang was pointing out that it was being ignored:

..\..\third_party\WebKit\Source\wtf/HashTraits.h(149,16) :  warning(clang): 'const' qualifier on reference type 'IteratorReferenceType' (aka 'typename OwnPtr<P>::ValueType &') has no effect [-Wignored-qualifiers]
        typedef const IteratorReferenceType IteratorConstReferenceType;
                ^~~~~~

BUG=82385
NOTRY=true

Review URL: https://codereview.chromium.org/170313006

git-svn-id: svn://svn.chromium.org/blink/trunk@167619 bbb929c8-8fbe-4397-9dbb-9b2b20218538