Remove unnecessary MemberHash<> templates.

Directly define the hash traits of all heap reference template types
(Member<T> - or Persistent<T>-derived) in terms of MemberHash<T>,
and avoid introducing ad-hoc templates for these.

Add missing (CrossThread)WeakPersistent<T> hash traits also.

R=
BUG=

Review-Url: https://codereview.chromium.org/2067223005
Cr-Commit-Position: refs/heads/master@{#400155}
2 files changed