HashTraits<RawPtr> uses 'nullptr'

It is now agreed in blink that 'nullptr' is used
to represent a null value for smart pointers.
This patch makes HashTraits<RawPtr> use 'nullptr_t'
as 'EmptyValueType' making it consistent with
HashTraits<RefPtr> and HashTraits<OwnPtr>.

Review URL: https://codereview.chromium.org/185903002

git-svn-id: svn://svn.chromium.org/blink/trunk@168520 bbb929c8-8fbe-4397-9dbb-9b2b20218538