Fix HashTraits types for OwnPtr

HashTraits should not use the OwnPtr's template argument type directly - instead it should use OwnPtr::PtrType and OwnPtr::ValueType types, as otherwise it defines some types wrongly in case the OwnPtr is handling an array.

Review URL: https://codereview.chromium.org/159043002

git-svn-id: svn://svn.chromium.org/blink/trunk@166972 bbb929c8-8fbe-4397-9dbb-9b2b20218538