Move WTF::HashTableDeletedValueType into its own header.

It was in wtf/RefPtr.h, but this it was there for historical reasons and
this is now a bizarre place for it to be. Including this header in
RefPtr.h is sufficient for the build to work because all places which
use WTF::HashTableDeletedValueType already include RefPtr.h transitively.

BUG=

Review URL: https://chromiumcodereview.appspot.com/19508002

git-svn-id: svn://svn.chromium.org/blink/trunk@154405 bbb929c8-8fbe-4397-9dbb-9b2b20218538