Declaring the weak_ptr_factory in proper order.

Cleaning up weak_ptr_factorydestruction order in "src/ash" module.
WeakPtrFactory should remain the last member so it'll be destroyed and
invalidate its weak pointers before any other members are destroyed.

BUG=303818

Review URL: https://codereview.chromium.org/497733002

Cr-Commit-Position: refs/heads/master@{#293450}
6 files changed